CPU/PSU fans spin for quarter of a second

So I just built a new rig with a friend, specs are

asus p7p55d - mobo
intel i5 2.66ghz quad core - processor
2x2gig DDR3 corsair dominator - ram
750w Corsair TX - psu

Upon putting it together yesterday, a simple boot with only the motherboard plugged in would produce 1 flash on the mobos CPU_LED accompanied with one beep. No fans would spin, and nothing would boot.

Plugging in a graphics card (ati 5850) would create a less audible beep, so I immediately jumped to believe it was the PSU. I plugged in my own psu (ocz 700w gamexstream) to the motherboard and success - it powered up. Having think I narrowed down the problem, to be sure I plugged back in the old power supply. But for some reason with the old PSU in the fans would spin for 1/4 a second then die, unlike before. Nothing would start up still.

So I exchanged the psu today for a new one (exact same one), and they tested it in house and it worked. At home, plugging the new PSU into the motherboard and attempting to boot, I only get the same fans spinning for 1/4 a second accompanied with the CPU_LED flash.

That CPU_LED, regularly, should only be on for a second or less on a regular boot. It's more or less a diagnostic light, but the problem is the system can't even boot on so it shuts off. There's a VGA_LED etc im sure many of you know this board better than me.

When it ran off my power supply, I was able to get it to the bios with no problem. I'm lost what to think here is the issue .... any ideas?