This is the second article in Your Computer Health series. For simplicity, I do not distinguish between viruses, worms and Trojans in this article.

What is a Virus?

A computer virus is a computer program that is designed to spread from one computer to another and to interfere with computer operation without the permission or knowledge of the user.

What they Do?

A virus might corrupt or delete data on your computer, use your e-mail program to spread itself to other computers, or even erase everything on your hard drive. Some viruses are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages on your screen. Even these can create problems for a computer user. Viruses typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes and data loss.

How they Spread

Viruses install themselves on your computer by exploiting security weaknesses. Computers which are not connected to the Internet or any network can receive viruses from a removable medium such as a floppy disk, CD, or USB drive, if they contain malicious code.

Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. Viruses can be disguised as attachments: funny images, greeting cards, audio or video downloads over the Internet or any number of other attachments in email messages. That is why it is essential that you never open an email attachment unless you know who it’s from and you are expecting it, nor download a file from an un-trusted source. Viruses can also be easily spread by instant messaging messages.

How to Prevent your Computer from Infection

Windows XP does not include antivirus software, so you need to download and install an antivirus program, such as Windows Live OneCare. You can get a 90 days free trial.

Microsoft also offers the free Malicious Software Removal Tool to help clean your computer if it should become infected. However, the Malicious Software Removal Tool cleans only a limited number of viruses and cannot prevent virus infections. Therefore, you still need to install antivirus software.

If you are not sure which antivirus software to choose, read the top 10 AntiVirus Software Review 2008. Installation of any of them will help.

If you use a single computer for home or non commercial use, you can take advantage of a free offer from AVG Anti-Virus. It is trusted by 80 million users.

The Free Edition offers basic protection against viruses, spyware, adware and identity-theft and it is compatible with Windows Vista and Windows XP.

Simply go to AVG Free Edition and download the software. Once you downloaded the software, follow the simple steps to install it on your computer. You will see the AVG icon  appearing in the tray (on the bottom right corner of your screen).

AVG will automatically download updates everyday (unless you change the settings). Make sure the icon in the tray doesn’t appear like this:  . This indicates that you may have an outdated database and you will need to download updates, or one of the security components requires your attention.