What is the best free AntiVirus software?

[IDKFA]

I've been using AVG for a few years, and while they're reliable (it's a loss-leader for their big powerful enterprise antivirus program) the marketing has been getting a bit pushier lately - not as bad as ZoneAlarm pushing paid versions, but still not as pleasant as it once was - they maintain daily updates with the latest badware updates.

Word of warning, I've heard things about it not playing well with (if I recall) Source engine games, so it may prove necessary to kill AVG before a bout of Team Fortress.