Hey, I have Windows XP, and after I installed a graphics card to replace my integrated graphics, i found that full screen games randomly minimize. A popular cause of this is AVG9.0 but I don't have AVG, and nor are any extra processes running apart from Svhost and a few others. When it minimizes there is no popup blocker or w/e sound, and it NEVER happened on my integrated graphics, but im not going to switch back because this is less of a problem than having 10-20 fps on some maps.
It almost certainly happens only during an 'action' scene in the game, not when im running around the map, although that did happen once. I've got only PCI slots (looking to upgrade PC, but in case i dont id like to fix this) so the best card i could find is an HD 4350, I've got a dell dimension 3000 atm, with a measly 250W PSU, which is 4-5 years old. However the graphics card is only meant to use 25W @ load, surely only a little more than integrated so im unsure if power issues are the problem.
I've gotten the blue screen 'critical error' once, but the game has minimized atleast 100 times.
To have a game minimize, but not crash, suggests that some other program is requesting windows for top/screen access. This could be a Windows update notification, an antivirus notification, or some kind of related "event".
If you're not using AVG, what anti-virus ARE you using?
It could even be minor things like a background Instant Messaging program, Facebook notifications, or something along those lines.
You might also want to make sure you're not inadvertently tapping the "Windows" or "Start" key on your keyboard if you have one. This could bring up the desktop, thus minimizing your game.