Program Windows Randomly Becoming Inactive?

So I'm having this irritating problem where, when I'm running any program fullscreen, it will randomly "minimize" every few minutes and go back to the desktop as if I'd alt-tab'd out of it. Problem is, I didn't. I relate the problem to watching your favorite movie, only the screen turns off every few minutes because that's just what it feels like doing. This gets especially annoying when gaming.The problem exists with windowed programs as well, although to a slightly lesser extent. The window randomly becomes "inactive" as if I'd clicked out of it. This disrupts things like typing and playing flash games (this isn't in the games section because the problem persists for ANY program run, it's just that a lot of these programs happen to be games).I've tried cleaning up my disk drive and cleaning out my registry, and I've restarted my computer twice I think. This problem happens regardless of background programs, including Skype and Steam, and, most importantly, no error message or notification of any sort appears when this happens.