(I) Computer Virus self-replicating computer program that interferes with a computer's hardware or operating system (the basic software that runs the computer). Viruses are designed to replicate and to elude detection. Like any other computer program, a virus must be executed to function—that is, it must be loaded from the computer's memory, and the virus's instructions must then be followed by the computer. These instructions are called the payload of the virus. The payload may disrupt or change data files, display a message, or cause the operating system to malfunction.

(II) A Computer Virus is a computer program that can destroy information on your workstation. Similar to a biological virus, a computer virus can reproduce itself by attaching to other files, usually executable programs. When isolated ( Unexecuted, such as in a compressed file), computer viruses are not dangerous, but when they opened, they can create havoc.

(III) A computer virus is a program that requires a host in order to make copies of itself on computer disks. Viruses may infect (copy to, and spread from), program files, programs in disk sectors, and files that use macros. The ability to self-replicate distinguishes viruses from programs that do not, and this parasitic nature is neither an accident, nor a computer glitch. All viruses are created by people who know how to write computer programs.

(IV) A computer virus is a piece of software that has been written to surreptitiously enter your computer system and "infect" your files. Some viruses are benign and won't harm your system, while others are destructive and can damage or destroy your data. Typically a computer virus will replicate itself and try to infect as many files and systems as possible. If your system is infected, when you save a file to a disk you will probably infect the disk, and in turn whoever uses that disk will infect their system. As you can see, it's a vicious cycle, not unlike the viruses that plague us humans. New computer viruses are being written all the time, and it's important to understand how your system can be exposed to them, and what you can do to protect your computer 

In order to be classified as a Virus, a suspicious file must have the following characteristics.

• Replicates itself.

• Attaches itself to other executables.

A virus is a piece of computer code that attaches itself to a program or file so it can spread from computer to computer, infecting as it travels. Viruses can damage your software, your hardware, and your files.

The first theories about the possibility of creating a self-replicating program date back to 1949, and experimental viruses were first programmed and tested in the 1960s. They got their name in 1984, when a university professor used the term "virus" to describe them, because like a biological virus, a computer virus is small, makes copies of itself, and cannot exist without a host. When personal computers became popular, PC viruses began to appear (in 1986-1987), at first intended as jokes, or developed for research or demonstration purposes.
 

Virus Code written with the express intention of replicating itself. A virus attempts to spread from computer to computer by attaching itself to a host program. It may damage hardware, software, or information.

A virus reproduces, usually without your permission or knowledge. In general terms they have an infection phase where they reproduce widely and an attack phase where they do whatever damage they are programmed to do (if any). There are a large number of virus types.

Most viruses do their "job" by placing self-replicating code in other programs, so that when those other programs are executed, even more programs are "infected" with the self-replicating code. This self-replicating code, when triggered by some event, may do a potentially harmful act to your computer.

Another way of looking at viruses is to consider them to be programs written to create copies of themselves. These programs attach these copies onto host programs (infecting these programs). When one of these hosts is executed, the virus code (which was attached to the host) executes, and links copies of itself to even more hosts.

Similar to viruses, you can also find malicious code in Trojan Horses, worms, and logic bombs. Often the characteristics of both a virus and a worm can be found in the same beast; confusing the issue even further.

A worm, like a virus, is designed to copy itself from one computer to another, but it does so automatically by taking control of features on the computer that can transport files or information. Once you have a worm in your system it can travel alone. A great danger of worms is their ability to replicate in great volume. For example, a worm could send out copies of itself to everyone listed in your e-mail address book, and their computers would then do the same, causing a domino effect of heavy network traffic that would slow down business networks and the Internet as a whole. When new worms are unleashed, they spread very quickly, clogging networks and possibly making you wait twice as long for you (and everyone else) to view Web pages on the Internet.

Worm : A subclass of virus. A worm generally spreads without user action and distributes complete copies (possibly modified) of itself across networks. A worm can consume memory or network bandwidth, thus causing a computer to stop responding.

Because worms don't need to travel via a "host" program or file, they can also tunnel into your system and allow somebody else to take control of your computer remotely. For example, the recent MyDoom worm was designed to open up a "back door" on infected systems and use those systems to attack Web sites.

Steps to Prevent from Worm:

• Use a firewall.

• Update your operating system and the software that you use. (Use Windows Update to update all Microsoft products automatically.)

• Use antivirus and antispyware software, such as Microsoft Security Essentials, a free download from Microsoft.

• Be careful with e-mail attachments and links on Web sites.

• Use a standard user account instead of an administrator account.

Examples of Worm:

The original computer worm was (perhaps accidentally) unleashed on the Internet by Robert Tappan Morris in 1988. The Internet Worm used sendmail, fingerd, and rsh/rexec to spread itself across the Internet.

The SQL Slammer Worm founded in 2003 used vulnerability in Microsoft SQL Server 2000 to spread itself across the Internet. The Blaster Worm also founded in 2003 used vulnerability in Microsoft DCOM RPC to spread itself.

The Melissa worm founded in 1999, the Sobig worms founded in 2003 and the Mydoom worm founded in 2004 all spread through e-mail. These worms shared some features of a Trojan Horse, in that they spread by tempting a user to open an infected e-mail attachment.

Mydoom also attempted to spread itself through the peer-to-peer file sharing application called KaZaA. The Mydoom worms attempted a Denial of Service (DoS) attack against SCO and Microsoft

Update your operating system and Web browser software. Your operating system (like Windows or Linux) may offer free software “patches” to close holes in the system that spyware could exploit.

Download free software only from sites you know and trust. It can be appealing to download free software like games, peer-to-peer file-sharing programs, customized toolbars, or other programs that may change or customize the functioning of your computer. Be aware, however, that some of these free software applications bundle other software, including spyware.

Don’t install any software without knowing exactly what it is. Take the time to read the end-user license agreement (EULA) before downloading any software. If the EULA is hard to find — or difficult to understand — think twice about installing the software.

Minimize “drive-by” downloads. Make sure your browser security setting is high enough to detect unauthorized downloads, for example, at least the “Medium” setting for Internet Explorer. Keep your browser updated.

Don’t click on any links within pop-up windows. If you do, you may install spyware on your computer. Instead, close pop-up windows by clicking on the “X” icon in the title bar.
Don’t click on links in spam that claim to offer anti-spyware software. Some software offered in spam actually installs spyware.

 
Install a personal firewall to stop uninvited users from accessing your computer. A firewall blocks unauthorized access to your computer and will alert you if spyware already on your computer is sending information out.

Adware is a type of software that is downloaded to your computer to show you advertisements. These advertisements may take many forms, from relatively noninvasive banners within a program, to very invasive pop-up windows that come up regardless of that you are doing in the foreground.

How Do You Get Adware on Your Computer?
You normally download adware without knowing about it, since no one usually wants to see advertisements whenever they run a program. So then how does adware get on your computer? Adware often piggybacks on other program downloads that you do want. For example, a lot of free software (freeware) programs have adware associated with them. When you download freeware you want, such as a peer-to-peer file-sharing program like Kazaa, you are also downloading adware that is packaged with it. That way, when you run the freeware, you are also running the adware in the background, which is creating advertisements on your computer.

.Function
Adware delivers ads to an infected computer in a variety of manners depending on the type of adware it is. Most adware functions only to create revenue and serve ads to unsuspecting computer users.
 

Types
Adware delivered ads can be delivered as pop-up windows, forced links within browser windows, browser redirects or toolbars that take up space in a browser.
 

Effects
Adware can slow a computer and affect the way certain programs work. The most likely to be affected are Internet browsers. Some adware may collect browsing information to deliver more effective ads to the infected computer.

Significance
Adware increases with increased Internet use and can often damage a computer user's experience with his machine. The ads delivered could be adult in nature, which could offend some or shock children.

Prevention
Adware usually installs onto a computer through the use of another program or through the click of a link. Carefully inspect options on downloaded software and be sure to run scans with programs such as Lavasoft Ad-Aware.

Identification
To help in preventing adware and identify possible adware programs, be sure to watch additional options in downloaded software. Any software that tries to install other than the software you downloaded is likely adware. The best way to identify an existing infection is with a virus or adware scan.

The term "malware" encompasses Trojans, adware, spyware, keyloggers, hijackers, dialers -- things of that ilk -- as well as viruses and worms. Malware stems from malevolent, malicious or malignant

Malware, short for malicious software, is software designed to infiltrate a computer system without the owner's informed consent. The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code. The term "computer virus" is sometimes used as a catch-all phrase to include all types of malware, including true viruses.

Types of MalWare:

Malware is a group term for anything malicious that can run on your computer, such as viruses, spyware, adware, trojan horses, and rootkits.  

Just as the mythological Trojan Horse appeared to be a gift, but turned out to contain Greek soldiers who overtook the city of Troy, today's Trojan Horses are computer programs that appear to be useful software, but instead they compromise your security and cause a lot of damage. A recent Trojan Horse came in the form of an e-mail that included attachments claiming to be Microsoft security updates, but turned out to be viruses that attempted to disable antivirus and firewall software.

Trojan Horse A computer program that appears to be useful but that actually does damage.

Hides things that are installed on your system, and conceals activities of other applications on your system, allowing those applications to do many things without your knowledge.

Some viruses have the ability to attach themselves to otherwise legitimate programs. This attachment may occur when the legitimate program is created, opened, or modified. When that program is run, so is the virus. Viruses can also reside on portions of the hard disk or floppy disk that load and run the operating system when the computer is started, and such viruses thereby are run automatically. In computer networks, some viruses hide in the software that allows the user to log on (gain access to) the system.

Virus writers range from researchers, to pranksters, to malicious vandals. The typical virus writer is an otherwise intelligent male, between 15 and 23 years old. He may be bored, curious, or intent on doing forbidden things, just to frighten others. Some belong to organized virus-writing groups (usually short-lived), and those in the group often respond to peer pressure, trying to outdo the others. Whether in a group or not, some get satisfaction from the challenge, while others think of themselves as rebels against the "system."

Tip:  Never open anything that is attached to an e-mail unless you were expecting the attachment and you know the exact contents of that file.

• If you receive an e-mail with an attachment from someone you don't know you should delete it immediately. Unfortunately, you're no longer safe opening attachments from people you do know. Viruses and worms have the ability to steal the information out of e-mail programs and send themselves to everyone listed in your address book. So, if you get an e-mail from someone with a message you don't understand or a file you weren't expecting, always contact the person and confirm the contents of the attachment before you open it.

• Other viruses can spread through programs you download from the Internet or from virus-ridden computer disks that you borrow from friends or even buy in a store. These are less common ways to contract a virus. Most people get viruses from opening and running unknown e-mail attachments.

• Back up important files on DVD/CD or any other Storage Devices on a regular basis, so that they can easily be replaced if a virus wipes out your hard drive. You can also used many types of Software available in the market, which is automatically take the backup of your important documents/files or other related material as per Schedule finalize by You.

• Install the newest antivirus software on your computer, such as Quick Heal Antivirus , Symantec Norton Anti-Virus, McAfee  (See Antivirus Gallery) Virus Scan for Windows, or Virex for Mac's.

• Update your Anti Virus software regularly for updates that will protect against viruses, Spyware, Addware, Worms or other threat which will infect your PC.

• Always using Microsoft Internet Explorer 7.0/8.0, Mozilla Firefox or Any other reputed Internet Browsers which will also maintain your Security, Privacy and securing to you from the Hackers when you are surfing on Net.

• Always download Freeware software from reputed Website like http://www.cnet.com, http://www.download.com