|Some detail of the hardware would be helpful. What brand & Model is the motherboard? The model number is usually located adjacent to ram slots or somewhere near outer edge of the board. Are the optic & Hard drives both IDE and are they on the same ribbon cable? Do you have a spare cable you can try? Is the cable fully seated in the motherboard?|
What brand/model & specification are the two powersupplies you have tested.
Check the tops of capacitors on the motherboard. Any signs of bulging or leaking?
If the system is a brand-name thing then what make/model?
Goin' Fishin' (Some day)