I have exactly the same problem. Same VGA and OS. You didn't give much information, but I've noticed that it happens regularly, every rounded hour, like 1PM,2PM,3PM... And it's not 1:01, it's 1:00, 2:00, like this. It's so weird! I think it has something to do with XP actualizations, but I don't know exactly nor do I know a solution. I'm hopping someone does.
System Specs: Core 2 duo E4300@3GHz, 2 Gigs kingston ddr800@834, 7600GT, XP pro SP2, 2 Seagate HDDs, Seventeam PSU 550W.
Noya, I think you should give the crowd more information about you problem.
P.S. Maybe it is some problem with nvidia drivers? I don't remember having these problems before the last ones (but it's been I while since I installed it, so I can't reverse the OS. I'll have to look for older drivers. Anyway, let's wait for answers from the knowledgeable ones.
Hmm...that's really odd, I haven't heard of something like that happening regularly...
The simplest (and probably not correct) thing I can think about is that you're accidentally hitting the windows key, which brings you straight to your desktop, minimizing your game. Otherwise, I don't know why the game would randomly/regularly minimize.