Windows 7 Windows crashes during games

Hello everyone, i recently did a clean install of 64 bit windows 7 RC. But when i play a game like, counter strike source or call of duty in full screen it crashes after about 30 minutes. My pc doesn't overheat and i recently cleaned out the heat sinks. My laptop is a Dell Inspiron 1520. I have not tested it to see if it crashes while only full screen programs, but thats not the point because i have a cod4 CG match in 2 days and i need this fixed!! Thankyou.