Motherboards frequently shut down as you describe due to cpu overheating.  The cpu cooler fan must have heat conducting paste applied.  Mine shut down in about 5 seconds without paste.  If you have applied paste already, I would start by removing all plug in cards (PCI cards, etc.), all but 1 memory chip, disconnect all cd, dvd, floppy drives, secondary hard drives.  If the video is not on the motherboard, remove the video card as well.  I think the computer should boot without a monitor, video card, or even a hard drive.  Then try to reboot.  You will be able to see if the power stays on, and if it does, you can power down and add the video.  Add back one component at a time to see which component might be bad.  If power doesn't stay on, try a different memory chip.  After that, you have to question the new motherboard, power supply, and cpu.    New doesn't always mean it's working properly.
Paul Hachmeyer
      
________________________________________
From: Personal Computer Hardware discussion List [[log in to unmask]] On Behalf Of Vanessa H. [[log in to unmask]]
Sent: Saturday, May 02, 2009 8:32 AM
To: [log in to unmask]
Subject: [PCBUILD] Newly Rebuilt PC turns off.

Hello all,

                I need a little help. I have built 5 PC's in the last 8
years and do minor repairs for friends, so I'm not a complete novice.  But
recently my son's PC would not turn on.  I decided it must either be the
power supply or the motherboard.  The first thing I replaced was the power
supply because it was the least expensive.  Well, that did nothing.  With
the new power supply in place, the PC was still completely dead.  I then
purchased a new motherboard, processor and ram.  After getting everything in
place, the PC turns on only briefly.  I mean it literally only runs for
about 5 seconds.  All the fans come on and it looks like it's about to start
up, but it turns off.  I tried unplugging all the wiring and starting from
scratch, but I still only get about 5 seconds before it all goes dead.  It
seems as though something is shorting out, but I don't know what?  Please
help!
Kind regards,
Vanessa H

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