This does not happen to ALL computers running AVG 9.0, but it can happen to some.
I don't know the exact triggers, however I have been reading AVG forums, and based upon what they have said and my own experiences, it appears that any OS running AVG 9.0 that was upgraded from 8.5 (using the link within the AV program itself, instead of downloading fresh from a browser), can mess with your full-screen games.
Every 30 minutes like clockwork, your full-screen games may minimize unexpectedly. I know from personal experience that this happens to WoW and Istaria both, and I suspect it would happen to anything fullscreen. I've also read user accounts on the forums that it can happen even with non-fullscreen apps like browsers, notepad, etc, where the window will lose focus and if you are typing, the window you were typing in will suddenly stop receiving input.
If anyone is getting this problem and are running AVG 9.0, so far the only thing I can deduce is that you should download the full installer from free.avg.com, and then cut your internet access (unplug the cable or something) and fully uninstall AVG and then install it fresh from the package you downloaded.
My Windows7 machine doesn't have these symptoms, likely because I installed AVG 9.0 from scratch with a fresh OS install, while my other computer is still running RC1, which had 8.5. I upgraded it by clicking the button in the popup window. Apparently the upgrade/install package you get through that means, fails to remove everything from the previous version, and the notification window (the one that was trying to tell you that you should download 9.0) still pops, but is a very tiny window that only appears for a split second, every 30 minutes.
This kinda sucks for Istaria, because of the whole CTD bug when trying to restore full-screen mode... just thought I'd let people know ahead of time. This appears to have only started since the AVG update/patch Tuesday morning.