So-called anti-virus software has never done anything but screw up my computer. So, I just don't run any.
I know, I've got a deathwish, but you know what? I wind up reformatting to clean up my computer about every 3-6 months anyway (I love my CDRW for backing up files I want to keep!) because it's the only way to clean up all the crap Windows creates. So if I got a virus that walloped my machine, I'd just shrug and reformat a little earlier than normal. Here are my stats for numbers of times my computer's been screwed up by two culprits:
anti-virus software: 4+
viruses: 0
hmmm... interesting trend...
Farscry