I was using AVG for a while:
http://www.grisoft.com/us/us_index.php It's pretty good, I go download it whenever I find that someone doesn't have an Antivirus program. It was fully compatable from day 1 with Win XP SP2, which is nice.
But Spybot 1.3 is a must have. Its very good at purging junk from your hard drive.