What is Antivirus
First, lets start off with how a dictionary define the term "computer virus"
(Computers) a program or segment of program code that may make copies of itself (replicate), attach itself to other programs, and perform unwanted actions within a computer; also called {computer virus} or {virus program}. Such programs are almost always introduced into a computer without the knowledge or assent of its owner, and are often malicious, causing destructive actions such as erasing data on disk, but sometime only annoying, causing peculiar objects to appear on the display. The form of sociopathic mental disease that causes a programmer to write such a program has not yet been given a name. Compare {trojan horse[3]}. [PJC]
To stop such viruses from doing any harm you install an Anti-virus software on your computer, and update it regulary. Most such applications got auto-update features, and thankfully computer distributors have started to include antivirus software with the computers they sell.
An important aspect to viruses is that in addition to revealing information about yourself, so-called spyware applications, many are used to do harm to a third party. Without sufficient anti-virus your computer may be used to send so called SPAM-messages (also known as: junk mail), or to bring down a bank using a Distributed Denial of Service attack, try to break into the Pentagon, or otherwise involve you in activities you hopefully don't want any involvement in
Antivirus applications
The most common antivirus software come from vendors such as F-Secure, Symantec and Norman. And although these vendors provide excellent applications, the subscription for new antivirus definitions is usually not free. Now, since new viruses are released so often, you are in as much harm if you use an outdated version as if you don't have antivirus installed at all.
One free antivirus software is provided by grisoft. I've had great success with this application and it is what I suggest to whomever use Windows and ask for an antivirus application. Some other free antivirus application for windows, deriving from clamav for unix, is clamwin and clamav-win32, being Open Source and released under the General Public License (GPL)