I can’t full view full screen

I can’t view full screen, well, I can, but only a minute, then it resumes on to desktop. I have no clue what to do. I have all drivers up to date. Any suggestions?

XP Pro sp3
ATI Radon Xpress 1150
AMD Processor

Funny thing, Windows Movie Maker is immune to the problems. Could it be a virus; I have protection?