I'd throw a vote behind AVG as well, unless you're running XP64 (it's not free for XP64 users...it is otherwise for personal use, as far as I know.)
If you're like me and in that boat, Avast! is a decent little AV scanner as well. My big issue is that the icon for it is ugly (little blue-grey ball). But if it's free and it's not a Symantec product, I'm okay with that.