I had a problem with PC failing to boot. All fans running so PSU seemed OK and I checked all the voltages anyway with a meter. I had a POST indicator attached to the MB which indicated failure of the CPU so changed that but still no boot.
I finally discovered that there was no voltage at the power OK pin on the ATX supply connector. When you start the PC, the PSU powers up but, in case it takes a second or so for the supply to stabilise, it does not allow the CPU to operate. This is so the CPU only has a fully stable supply. When the PSU decides it is OK, it supplies 5v on the power OK pin (no 8) and the CPU starts. Without the 5v you wait forever.
My PSU did not deliver this 5v even though all voltages on all other pins were OK. Whether a fault of the power on system or maybe the voltages were unstable, I don't know.
I was a bit annoyed because i paid a lot of money for a high end PSU. A new (and cheaper) PSU solved the problem.
On a cheaper PSU, sometimes the power on 5v is just hard wired ie no checking of the PSU anyway so you would not get this probem (but more chance of a fried board).
Hope this helps someone.