Before you send it away, a few other things to check, both of which I have seen on client's computers:

1 - front of case case power or reset switches sticking. One fellow's computer had the reset switch stick, so the unit would power up for 10 seconds, then shut down. So, locate the switch wires for both switches where they connect to your motherboard. Power up the box, and as soon as there is power, YANK them quickly from the pin headers. If power stays on, there is your problem.

2 - Motherboard battery. If the battery has lost it's juice, then your CMOS/BIOS settings could have been lost. When the PSU comes on, the mobo checks the BIOS, doesn't know what processor or RAM frequency settings to use, what type of HD's to search for, which disk to boot from etc. If you can, get an electrical meter, and test the battery without removing it from the mobo (with the PSU off and unplugged.) It should read between 1.5 and 3 volts DC. Anything lower and the battery should be replaced. Ideally, you want to replace the battery with the power on and running, but that isn't always possible, like in your case. If that is the case, you wil have to go through the BIOS and set everythign back up how you had it. And before anybody says it, it doesn't matter that this is a brand new mobo. I have seen weak / dead batteries in brand new mobo's. It's not when you bought the board that is important, but the date that the battery was made. Who knows how long the battery may have been sitting on a shelf at the factory before it got put in your brand new mobo?

If those don't work, then as suggested earlier, try a different Processor and different RAM if you have them available.

Good luck.

Dexter...