Having a problem with my BACK up computer, which is CONFIGURED with a PC chips P53G motherboard and a dual core 3.0 gig Pentium processor. When I hit the power on switch, the power supply and processor fan come on for a second or two and then turn off. I tried switching to another known good power supply, bypassing the front switches and re-set switch, and even replacing the power cord and plugging directly into the wall outlet. My last change to this computer was to add a 1 gig memory chip, so I even tried removing that but all my troubleshooting has not solved the problem. The system is only 11 mos old, and since it is a back up I haven't used it that much but if anyone has any other ideas I might try or missed I would appreciate the help or suggestions ! My backup pc does the same. I get around it by holding the push-button for a second or two. If I hold the button too long it will shut off again. If I don't hold the button long enough it shuts off.Did you assemble this machine ? ?
If so were all MBoard standoffs used ? ?
Thermal paste applied correctly ? ?
Have you checked that all fans are operational especially the CPU fan ? ?Thanks for the RESPONSES! I did assemble the system, I attached the MB with whatever standoffs were present in the old case, I'm not sure if I used all the mounting holes that were in the MB, but the system did work for quite a while after I assembled it. Also when you hit the front power switch the processor fan and the power supply fan come on but just for a second or two. Some others have suggested disconnecting the drives and resetting the cmos which I have since tried to no avail. When I have done this and tried with just the MB and ATX connectors hooked to the MB it still does the same thing.Cheapest most effective test would be to borrow a known good PSU of the same wattage or greater overnight and swap it in there to see if it's the culprit...
If not Post back.Running for a few seconds and stopping is generally a power supply or thermal fault detected to protect itself.

Are all cooling fans correctly connected to the motherboard. CPU Fan, and sometimes a 80 or 120mm case fan which both have the extra fan speed monitoring encoder to measure fans RPMs and shut down the main board when fan motion is not detected or not up to par?

Already mentioned by Patio, but also suggesting what Patio stated from my own personal experience with this problem in the past. Biggest goof up by a customer I saw came in wth this issue was the CPU fan 3 wire cable plugged into wrong jack on motherboard. The CPU cooling fan was plugged into the jack normally used for 80 or 120mm enclosure case fan, and the CPU fan power /monitoring jack was there in the open with nothing plugged into it. So please check to make sure CPU fan is plugged into correct 3 pin jack on MB to eliminate this POTENTIAL problem. If you got these backwards and the 80 or 120mm quiet fan spins at too slow of a rate you can also get this issue.Great point Dave...i've had that happen as well.Thanks to Dave and Patio for suggestions, I had a local guru take a look at the desktop and he swapped the power supply again and bingo , problem solved. This was after I had taken a good power supply out of another of my working machines and had not solved the problem. The guru informed me that not all power supplies are created equal. When I saw the one he installed I'm still a little perplexed as it is a 250W and the one that was not working was a 550W Power Magic. I think the 250W was a Tiger, and it was a 20 pin. If any of you guys can recommend a decent PS that doesn't cost an arm and a leg, I'd appreciate it. Or maybe you just have to bite the bullet and spend upwards of $50. Anyway, thanks again for the help.Depending on your needs that price is a good starting point for a QUALITY PSU.
Keep in mind future needs such as SLI compatible if you're a gamer and want to run 2 vid cards; plenty of SATA connectors etc.
A few good brands would be Antec, OCZ, Raidmax, and the mid to high range CoolerMaster series.
PC Power and Cooling is a great site for PSU info along with the review section at Tom's Hardware.