First, I'm running Windows XP (SP2) on this machine:
Gigabyte GA-K8NF-9 Motherboard
Gigabyte GeForce 6600GT
AMD Athlon64 3200+
2x1gig mushkin DDR RAM
Western Digital 250gig SATA hard drive
Antec 380W PSU
I will have had this computer two years this spring. I put it together myself, and did not have any troubles with it until late October of this year. When my problems first started, my computer would freeze up randomly, anywhere. If I was playing any sort of 3d game, it would first freeze up (the sound would continue) and then I would get a long sound loop, which would then cut into a very short sound loop, and then it would restart.
I have not locked up anywhere out of a game in over a month, so I think I may have fixed that through one of the myriad things I have tried to figure out just what is wrong with my computer. One of the first (and probably most foolish) things I did was to reformat my hard drive. It didn't fix the problem. I have run Memtest86 and through seven passes it found nothing wrong with my RAM. After this I installed my second stick, and have tested the second, newer stick standalone with no improvement.
I know the problem is not heat - one of the things I feared at first when I opened the case and looked at it was that the layout of my motherboard was heating up my graphics card. The NForce chip on my particular motherboard sits right under the lip of the GPU. The heatsink off the chip heats up that corner of the graphics card quite a bit, but even with a floor-stand fan blowing straight into the case I still experience the crash.
It is not a graphics driver conflict - I have gone in with Drivercleaner and completely uninstalled my old drivers and installed the freshest ones available, to no effect.
No error messages or BSODs accompany this crash, even though I have disabled Windows automatic restarting. Nothing shows up in the event viewer, either.
The most irritating aspect of this problem is that, even though it is completely random (sometimes I can go for days without a crash), if it happens to me once, it will happen over and over again.
Example: I am playing Battlefield 2. Two hours in, my computer crashes. From that point forward, (maybe six hours or so time) I know it will be useless to try to play any more because it will crash within fifteen to twenty minutes of starting the game again.