New Build: Bad motherboard or CPU??

[Robbo323]

New PC I put together will only start to power up for 1/2 second and then shut itself down. LED in power supply lights up and CPU fan starts to spin then both shut right off. Tried everything in the "try this first" section: Build outside case, stripped to MB, CPU, HSF, 1 stick good RAM, screwdriver to power contacts - same problem. Popped battery to reset CMOS. Same problem.

(Strangely, being at a loss, I tried unplugging the extra 4 pins on the 24 pin power connector and powering up with just the 20 in. HSF stopped spinning in 1/2 sec as before, but LED remained on in the power supply(?) Wasn't expecting that.)

I'm stripped down to a MB & CPU with a known good PSU. How do I figure out whether its the CPU or the MB that I need to RMA??? Thanks for any help.

Specs:
AMD Athlon 64 X2 5400+ Brisbane 2.8GHz 2 x 512KB L2 Cache Socket AM2 65W Dual-Core black edition Processor
GIGABYTE GA-MA78GM-S2H AM2+/AM2 AMD 780G HDMI Micro ATX AMD Motherboard
Rosewill RP550-2 550W ATX12V v2.01 Power Supply
Kingston 1GB 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)
ARCTIC COOLING Freezer 64 Pro 92mm CPU Cooler

[Uncle Rico]

Does the red light next to the CPU fan come on?

[MaxRat]

I'd be betting on a bad board...very rarely do CPUs arrive bad...I have yet to get one...if the ram isnt good it will also power cycle in some instances...especially if the ram is 2.1v or higher...