i think you're all wrong...you need to:
take the entire computer apart, carefully
put each circuited component on a non-magnetic lint-free material (plastic is good)
clean the dust off of each component, especially fans
unplug everything that can be and plug it in again
remove the CPU, clean it off, buy some of that thermaltake heat creme, reapply it and reseat the CPU.
then:
back up your stuff
format your PC
reinstall windows WITH service pack 2
re install all of your drivers
re install programs and games
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look, it's going to take around 4-5 hours, it's a saturday or sunday job, very fulfilling, and if it doesn't sort out your problem i will gladly do it for you. (not)
but i'm dead serious, this will sort out your problem, and afterwards, your computer will feel like a phuckin' badass machine.