At work (IT support) we use AVG Free [http://free.avg.com/gb-en/homepage]. Works well enough, and doesn't interfere with too many games. Has rarely had issues, only once have I had to unistall it from a client PC, and even that was an instant reinstall, fixing the problem (update issues). One thing I advise is not letting it have its way with your PC in terms of add-ons, such as e-mail scanning and the toolbar. It is a bit intrusive and annoying more than actually disruptive, but I always avoid it.
Oh, and Nortan Anti-virus is a swear to most people in the IT support business. With me, it would constantly come up with an update screen, five minutes into using the PC. Not booting it, because then I could just boot it about five minutes before I plan on using it. No, usually after getting a short way into a game
Ping
The game has a fit and up pops Norton Anti-user
And some games have issues with minimising them then going back
Anyway, good luck with whatever you choose!
Matthew