To do a basic check out of the PSU:
Try to borrow a known good PSU. (Brand new, out of the box, untested PSU is not a known good one.) If you cannot do that, borrow a DMM to measure the voltages. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.
The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.
You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.
This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.
Well this computer wasn't mine it was my friend computer but know he bought a new one, so he asked if i want it and i told him yes. Is a pentium 4 3.00ghz, 1gb ram DDR, Geforce 6200 256mb DDR. i don't remember righ now the model/type of the motherboard but i'll find it later.
Well if the pc lights up but no beep or sound or anything I would guess that the atx power supply may not be connectedto the board or it may be faulty. Remember there are 2 power cables that connect to the motherboard one on the outer edge of the board and one small one somewhere close to the cpu. Make sure they are both connected.