At 3+GHz, the more usual is an overtaxed power supply. See 350Watt PSU's all the time that have collapsed under the strain. It's the cheapest part to replace in the set, but before all that you can DOWNSIZE the machine to no cards, no memory, no drives and see if the remaining CPU/HeatSinkFan (HSF), power supply, speaker will beep in protest. Then you can add one stick of RAM, a video card and try again...

Light reading about power supplies at http://www.tomshardware.com/howto/20021021/

Bob

Bob