What I Use: AVG Free
Does it work well? Yeah, I don't remember the last virus I had that caused me a problem.
I actually don't know why it's installed though, since I don't remember the last time I got a virus. XD
Pay: $0
Happy? Automatic updates are, in my opinion, really annoying, and cause a significant decrease in system performance during startup. Interface looks crap (but thank god they didn't try as hard as Norton or McAfee which were both failures), otherwise the actual virus detection and stuff is excellent.
Acquired:
http://free.grisoft.com