ACK! WHY MY GAMES CRASH?

OK, im running windows 2000 pro, asus p4s533, p4 2.0, 512 pc2100 ddr ram, creative audigy platium for sound, and a geforce 4ti 4600…recently my 3d games such as mohaa and rallycross challenge are dumping out to the desktop or cause an outright crash or reboot. any ideas? I recently wiped the hard drive and reinstalled EVERYTHING! All my drivers are fresh, so what gives? PLEASE HELP ME.