[adanthar]

Are you overclocking? If you are, it's got around a 100% chance of being related to that.

If not, it's likely memory, heat, CPU, motherboard in that order...maaaaybe a hard drive going bad, but I doubt it. For starters, try opening/removing the side of the case and putting a big regular fan right up against the PC; if it runs for two hours while that thing is blowing, it's heat. If not, buy a cheap memory stick, remove the existing memory and see if that works.