A clue the motherboard has suffered some damage. The problem with home repairs is the lack of diagnostics to tell us if it's a CPU, board or such. In this case I'd get a CPU and motherboard combo and move onto the new machine.
HP a345w W XP Pro. Orig. asked in XP forum no help. Thiking it might be hardware problem.
The plastic holder on heatsink broke. Put on a new heatsink & fan. Paste on heatsink. Tried to boot and fan on heatsink comes on for 2 seconds and shuts off.Tied new power supply and new harddrive. Still will not boot. HP support said take it to Best Buy. I am sure someone here will have the answer. Thanks for your help. Darrell