Used to use AVG, but they have problems with their free version update servers.
They have? I've been using Grisoft AVG for over a year (ye gods, maybe even two by now) and I haven't ever had an update server problem. The only problem that I had was a bug in the first version I used that, over time, would bring the system to a crawl in some games. Exiting the game and starting it up again would fix that. And then that bug was fixed. No problems since.