Sounds like an overheating/video. I agree with U235, go get a cheap card ot test for a bit to see if it's the video, or get some more cooling in there. A quick, simple test ot signify between a hard lock, and a software issue is, when it happens, hit the NUMLock key. If it flashes on and off when you press it, the computer is not "locked up", but the video has quit out/other problem. If the NUMLock light stays frozen on or off, then the PC is locked up, and you have generally a hardware issue, or an overclock/driver issue that is locking up the PC.