On my Windows XP partitions I'm using Avira Antivir Personal 9. Quite satisfied with it, best free anti-virus in my opinion. But I'm starting to use Linux (Ubuntu 9.04) a bit more lately.
I was using Nortan from quite a long time, but i am fan of McAfee, its one of the best that i have usedso far. there are many other which gives free service like AVG and AVASt that are pretty good too. Thanks