Shutdown after 20 seconds

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Hey everyone! I've just finished putting together my second build but
i seem to be having a few problems. The system seems to turn on just
fine but the whole computer will shut down after about 15-20 seconds.
I can enter the mobo setup screen but this doesn't seem to make much
difference. Originally i thought it was probably a PSU error so i
replaced it and still have no luck. I've so far tried re-orienting the
RAM, disconnecting hard drive and cd drive and trying to boot as well
as clearing my mobo CMOS. So far nothing has helped. Any help i could
get would be MUCH appreciated.

The setup as of now is:

Asus P5N-E SLI
Intel Core 2 Duo E6400
XFX GeForce 7900GS
Patriot 1 GB (2 x 512) DDR2 800
MasterPower 500W power supply
1x40 GB SATA
LiteOn DVD/CD-RW combo drive

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