about 90% time its a worm
about 5% its a bad driver (video)
A virus scanner running in backround can do it (Norton especially)
Your install CD (cough) could be corrupted or infected
However,
in your case, trouble even loading new windows install could also mean CPU overheating or not seated or too much goop, or wrong bios settings - RAM timings or CPU - RAM voltage
Perhaps Insufficient voltage to mobo/vidcard (PSU)
Based on your info, I lean towards bad hardware (mobo)/BIOS corrupted or setting probs.
You could try reflash bios to older vers.
You should look in Dev Man and see whose chipset drivers are loaded - MS or Nvidia
Is your PC fast in safe mode??
Is PC fast with failsafe defaults set in bios?
While it prob wont help here, you could download killbox and stop services one by one to see whats going on
http://www.bleepingcomputer.com/files/spyware/KillBox.zip