Hi I have a Dimension 5100 running Windows Media Center.
My problem is that after I have shut down th PC via the Windows 'Turn Off' process the machine will not switch on again via the the power button. The only way to get the machine on again is by disconnecting the power lead for 10 or fifteen minutes and then reconnecting.
Odds are roughly 70 : 30 that it's motherboard : power supply. The only conclusive test is substitution with a known-good PS or MB. You don't have access to a working 5100 you could 'borrow' the PS from, do you? If it does the same thing with a substitute PS, the MB is faulty.
You do mean that you get no response at all from the power button until you've unplugged and waited, right? If you got a yellow power light but no other response, that changes the odds closer to even.
During one of these 'won't come on' times, remove the cover. If the motherboard light is OFF during this time, the power supply is bad.