Clamwin is the best antivirus program - if you are at least somewhat technically savvy.
Version to use: .88
I have used all the most popular commercial anti-virus program and they are almost all resource intensive applications. They will all slow your computer and give you weird popups about stuff you don't care about.
The basic problem with current versions of clamwin is that they won't do "realtime" scanning. So, this is a problem for people who 1) open files/downloads/attachments without first scanning them to see if they are safe 2) use virus rich technologies like ActiveX, Internet Explorer, Microsoft Office, and Outlook Express. I don't do any of those things, so I'm safe. Nikki is less safe, but I hide those things from her and don't let her use the computer very often :)