Well, since I'm not a hardcore PC gamer or programmer I will try to add what help I can from my casual experience. My computers only seem to bog down for a couple reasons. #1 is spyware/viruses, #2 is temperature, other than that I'm not sure about hardware problems. If you computer was running well before and then this happened then I'm sure it's some sort of malicious software. I've never had a hardware breakdown that allowed me to boot up.
I had a problem once, my computer is located very near the heating duct, and right at the beginning of the cold season here in Ontario I noticed some major performance problems. Stupid me, I checked my specs and my video card etc were running at 45 degrees celsius or more. I air blasted it out, moved it away from the duct, and haven't had problems since.
My girlfriend's laptop also had a problem where the standard antivirus that Compaq(shudder) includes somehow decided that Windows Framework 2.0 was bad news and deleted it. It would run for around 5 seconds, then freeze for 10, then run for 5, freeze for 10 etc. I hunted down the framework again and no problems.