I'm currently using Microsoft Security Essentials. Does a pretty good job.
I used to use AVG, but it sorta screwed with my games...
Then I went to Avast. That sucked. I had no control over what it did, and it would give me "access is denied" messages when I tried to end the process. Sega, I'm the administrator and this is my computer, you don't tell me what I can and can't do!
I uninstalled it, but it was a Sega to do. When I used the built-in uninstaller, it left lots of little pieces behind that I couldn't get rid of on my own, probably because I couldn't shut it down or just plain end the process and uninstall it properly. So I had to reinstall the whole thing, remove it from my startup programs so it wouldn't run when I turned my computer on, then use Revo Uninstaller to get rid of it all, and it still managed to leave pieces behind. Two months after the fact I was still finding folders labeled "Avast" on my hard drive; I'm pretty sure new ones were generated as time went on, too, not sure how... Reinstalling 7 was the only way I could get rid of it all. I'm never using that one again. o_o
After that, I went to Avira... not sure what I didn't like about that one, but I know I had reason to uninstall it. I think it didn't play nice with some of my programs.
Then I tried Microsoft Security Essentials. It's not bad. Unobtrusive, easy-to-use, free. I'm satisfied with it. That, along with sensible browsing practices and not downloading everything I come across keeps my system pretty safe.