I have a serious problem.

Sometimes I want to run a Dos game directly from windows. When the game enters fullscreen the game locks. I don't know when this problem started but I know for sure that it haven't been there always. Is there anyone else that had the same problem? I have:
Windows 2000 SP 4
Motherboard: ASROCK K7S8X
Graphic card: Sapphire Radeon 9800 PRO (tested with Creative Geforce 3 TI too)
Somebody please help me!!! :help: