What is the best free antivirus program?

I used to use AVG, but with their 8.0 update everything went to hell. Suddenly it was chewing through resources, flagging everything it shouldn't, and causing a major slowdown for startup time.

Avira is now my go-to program. It's small and for the most part it doesn't interfere with my normal operations. It does, however, do daily updates alongside a popup window asking you to buy the premium version. This alt-tabbed me out of a few games, and I was not happy.

At any rate, a google search will show you how you can get rid of the popups and/or make the virus definition updates run invisibly.

I've heard some people swear by Bitdefender, but I've never used that one myself, so I can't comment.

Jam it back in, in the dark.