I would recommend you follow the sticky post for "My PC won't start" (strip down to bare minimum, check RAM, etc). If that doesn't point you to the problem, here are some other ideas. Excessive dust/dirt build-up inside the computer could be the cause of the problem. If the fan on the processor heatsink isn't running or is running very slowly, I would check the processor first. The fastest way to determine if the processor is bad is to replace it or try it on another board. Excessive dust could have killed the power supply as well. When the dust works its way into the fans, they can stop spinning which will result in overheated components. If the fans are that bad, don't just clean them, replace them.