I can’t figure it out!

My pc always locks up when I play games. Either it locks up, crashes to the desktop or gives me an illeagal operation message. all of my drivers are up to date and support directx 8.1(the version I currently have installed). I can’t figure out what the problem is. Any advice would be much appreciated.

1.3ghz AMD Athlon
256mb DDR
Geforce2 mx400
Sound Blaster16