I built a new computer last night and when I went to power it up for the first time, it starts and runs for about 3 seconds, then shuts off.  I have built Intel machines before, this is my first AMD.  Here are the specs:  Asus M2N-SLI Deluxe Motherboard, AMD Athlon 64 x 2 5600 processor, 4 gig CORSAIR XMS2 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400, 2 500 gig Samsung HHD raid, Leadtec 8800 640 mb video card, and a 630 watt power supply.  Any idea where to start looking?  I took it apart twice and rebuilt with no luck.  Thanks!

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