Rebooting computer

hello there, ive got a Pentium 4 1500 mhz, with a Geforce2 in it… first i couldnt play games with OpenGL, i downloaded the detanator pack, now i can… but when i close games who need OpenGL, my computer reboots!! I also got it sometimes with IE, but almost everytime with OpenGL games, like RTCW and more… does anyone more has it?! Or does anyone has a solution for it?
RoGaH…