Avira AntiVir personally.
Regularly updated, very low memory footprint, not many false positives, unintrusive, and best of all... free. Does come up with a nag screen when you update, but if you use windows to deny running of notify.exe (or make it run minimised) it stops popping up.
AVG is OK too, but its a little too cautious for my liking, and pretty crap at detecting even kiddie scripts.