I use AVG Free, but since it's about to become AVG Not So Free, I'll probably either switch to a pirated copy of AVG or maybe try Avast. I think I have it set to auto-scan really early in the morning, and I have enough ram that it doesn't really cause me any problems anyway.

I hate Norton with a passion, as that's what my dad had on our old family computer, and it failed to clean up some that Panda's free online scan was able to get rid of. It never seemed to stop us from getting any viruses at all.