I have used Norton Antivirus, McAfee, AVG, and Prevx.
On one computer: nothing.
On the other computer: Prevx beta testing version (no longer available)
I've actually found prevx to be good, but maybe I think I've only had 1 virus affect my computer since I've had it. I installed it to get rid of a virus that was on my computer.
I'm not sure if Norton stops protecting the system after u stop paying, b/c I got a few viruses that Norton couldn't get rid of (not sure if it was when I was or wasn't paying).
I think I had AVG and it didn't do the job for me. I only get viruses like once every year and a half or 2 years.