Power On For Sec, But Then Turns Off

[CodeR3d]

PROBLEM
Okay so my computer randomly stopped booting up one day with no major system changes. The only thing it did is when you pressed the power buttom the lights would come on and all the fans would spin for half a second then immediately shuts off. Then if you press the power button again nothing happens you have to shut off the power supply then turn it on again to get the same result.

POSSIBLE FIX
So I assumed it was my motherboard that went bad and wanted to upgrade my computer anyway. I purchased the following Intel Core 2 Duo E7200, ASUS P5QL Pro, and G.SKILL 2x1GB DDR2 RAM. So I basically made a new system which I put together.

STEPS TAKEN
I then tryed to boot it up with the minimal required hardware which was the cpu, board, ram, video card, psu, and floppy. Guess what? Still does the same thing as before. A possible thing could be a faulty power switch on the case but it cant be that because I put a screwdriver to the pwrsw pins and it did the same thing. The switch also seems to function fine as before when pressed in. So that has been eliminated. This leads me to believe it must be an old component that was from my existing pc. The only major parts this could be that were connected before are the existing video card, psu, floppy (cant be this!). So it must be either the old psu or old video card? There is just nothing left to be faulty because I have tried each new memory stick one at a time in different slots and the new board power light is on when the psu is on. The new cpu was installed correctly and those are usually not faulty. I tried booting with hard drive connected with the same problem.

FUTURE STEPS
1. Try a basic PCI video card to see if its the one im using thats the problem
2. Try a new power supply if the same problem occurs after that

HELP!
I just spent a bit of money on this new hardware hoping to fix the issue but it has not. I really need some help with this one... Please give me some ideas or let me know if im on the right track.

THANKS

[mjc]

Bad power supply is the most likely cause...in fact, it is very probable that your original motherboard is fine.

A computer power supply is a complex critter that supplies several different voltages. One of the supplied voltages could go out and the PSU would partially function...but it wouldn't be able to provide the 'power good' signal that the motherboard needs to power up...so, it could be 'mostly dead'...the fans will attempt to start, etc, until the motherboard doesn't receive the 'power good' signal and shuts everything down, again.

[CodeR3d]

It was indeed the PSU... I upgraded to a PCPC 500W and all things are running smoothly so far.