I had a similar problem, eventually I replaced the mother board. If you go to a retail store like Fry's or one with a good return policy, see if you can't pickup a PSU and a MOBO. Try the PSU first, as it's relatively easier to swap out some cables, if that doesn't work, then replace the motherboard. Eventually, you will find the problem.
A word of caution, updating the old computer with a new motherboard isn't always practical. With a Socket 478, at least you have some future upgrade potential. But if in 6 mos from now you're hating this computer you might regret sinking $50 into an outdated system.
"To err is human - and to blame it on a computer is even more so." - Robert Orben