ClamWin is a very cool lightweight open source anti-virus program. This may not be the best anti-virus solution for beginners, but experienced PC power users should check this out. The download size of this application is around 15 MB. I really like that this application is so light weight and uses little system resources. One downside (if you can call it one) is this development does not yet support real time scanning. You can schedule automatic updates and scans though. ClamWin also creates nifty reports on your updates and scans. You can use it to scan individual files and it also integrates seamlessly with Outlook.
ClamWin is a Free Antivirus for Microsoft Windows 98/Me/2000/XP/2003. It provides a graphical user interface to the Clam AntiVirus engine