AVG has a beta of their free antivirus software availablef or download. This copy expires 11/30/2004, but by then the full version will be out. The new version is of course compatible with XPSP2 (like the non-beta free version).
This is kinda old news, but bear with me as I peek back into the land of the living...
Personally, I use NOD32... mostly because it's bloody fast and doesn't suck resources... AND it's actually pretty student-friendly... their Education price that I managed to get myself and 10 co-students worked out to $25 CDN for two years.
Compare a bunch of them at Virus Bulletin - http://www.virusbtn.com/vb100_award/about/index.xml