I held off upgrading to AVG Free 9.0 Virus protection for as long as I could...And now I see what the little voice was trying to tell me!!
There is a nasty little side-effect of the new and "Improved" version 9 which involves minimizing full screen every 30 mins. This is not life-threatening if you are playing a turn-based game or in something such as "For The Glory" or Hearts of Iron" that isn't terribly time critical. However, tempers get a little frayed when you are just pulling up your FW190 after a swooping dive in IL2 and the screen minimizes! Or fighting desperately for your life against some hideous, evil-smelling barbarian with an ice-axe in "Mount & Blade" Warband. You return to full-screen mode to behold a ten-foot deep smoking hole in the landscape, or your horribly mutilated corpse.....not the perfect end to a lousy day.
If you are experiencing the symptoms described in this THREAD after upgrading to AVG 9.0 then fear not noob, for there is a fix! Which involves uninstalling AVG including checking the box "DO NOT SAVE YOUR SETTINGS" then downloading and installing avg_free_stf_en_90_707a1765.exe as per this post by JANA.
This certainly worked for me and now I'm back to happily playing away with no frustrating minimises whilest eagerly awaiting my copy of Windows System 7 and all the joyful teething-troubles that that much-maligned gem will involve!