Clamwin is a free open source antivirus for the Windows Operating System, it’s supported on Windows 98/ME/2000/XP/2003. It features high detention rate for viruses and spyware, scanning scheduler, automatic download of regularly updated virus database, standalone virus scanner and right click menu integration to Microsoft Windows explorer, and an add-in to Microsoft outlook to remove virus infected attachments automatically.
One of the things this anti-virus lacks is an on-access real-time scanner, you need to manually run the scan to catch the viruses or spyware, this the reason I do not use Clamwin antivirus myself, an antivirus that lacks a real time access scanner is not complete, but is worth to keep this antivirus on the watch, because as soon as they add an real time access scanner, it will be an awesome antivirus software.
I have always trusted AVG Free Edition. It has never let me down in the past 2 years. The closest is Kaspersky.