You should check your hardware.
...and make a Backup right now!
The first thing to do (after the backup) would be monitoring the Systems temperatures. SpeedFan may help you with this. Install it, set it up to log your temperatures to a file and then do whatever you need to do to kill the machine. Maybe it's just overheating and the CPU (or another component) resets itself?
If this does not work boot from any fairly modern linux live CD (like ubuntu) and selevt "check memory" at the boot prompt. It will run memtest+86 which checks your ram. Let it run for a while (over night?) and complete 2-3 complete cycles.
Everything else will be hard to check. I'd reinstall windows at this point and see if the problem goes away. Otherwise: Change each component step by step (if you have spares).