The following is cut & pasted from a previous post.
Try to verify (as well as you can) that the PSU works. If you have a multimeter, you can do a rough checkout of a PSU using the "paper clip trick". You plug the bare PSU into the wall. Insert a paper clip into the green wire pin and one of the black wire pins beside it. That's how the case power switch works. It applies a ground to the green wire. Turn on the PSU and the fan should spin up. If it doesn't, the PSU is dead.
If you have a multimeter, you can check all the outputs at the back of the main power connector. Yellow wires should be 12 volts, red 5 volts, orange 3.3 volts, blue wire -12 volts, purple wire is the 5 volt standby. The gray wire is really important. It sends a control signal called something like "PowerOK" from the PSU to the motherboard. It should go from 0 volts to about 5 volts within a half second of pressing the case power switch. If you do not have this signal, your computer will not boot. The tolerances should be +/- 5%. If not, the PSU is bad.
Unfortunately (yes, there's a "gotcha"), passing all the above does not mean that the PSU is good. It's not being tested under any kind of load. But if the fan doesn't turn on, the PSU is dead.
I've checked all the connectors and molex. All the volatges r gud (yellow +12, red +5). So do I conclude that the board is fried? keeping in mind that my replacement psu was a dud, i'm gonna try with another psu as well.
Maybe your front panel power switch has given up the ghost? Try removing its header, and mometarily shorting the two pins revealed, say with a small srewdriver blade - if the board powers up, you've found your culprit!