This question gets asked frequently here. The most popular choice for a free antivirus is probably AVG Free Edition:
http://free.avg.com/g...d-avg-anti-virus-free
It should be remembered that anti-virus software is only one part of a full security suite. You should also install a decent firewall, such as Zone Alarm
http://www.zonealarm....ity-free-firewall.htm
and some anti-malware software, such as SpyBot S&D
http://www.safer-networking.org/en/download/
and/or AdAware (left hand link):
http://www.lavasoft.com/single/trialpay.php
However, having typed all of that lot out, I should also point out that Microsoft Security Essentials is gaining quite a large fan base around here:
http://www.microsoft.com/security_essentials/
Chris