A Problem with the PSU or the Motherboard???

I have recently purchased a diablotek case and i connected all the cables (the PSU to all the motherboard ports, the ribbon cables, and the power switch and led's) problem is that when i try to start it up it will power on for a split second and then it will just shut off. My mother board has one 20pin cable connector and has one 4pin connector and they are both plugged in accordingly, the 12volt molex cables are plugged into 2 250gig seagate hardisks and a dvd-rom. What is wrong and why is it powering up for a split second? any help would be appreciated

Thx,

gnar

Two things I can think of. Thermal Shutdown of the CPU; is the CPU or HSF seated corrrectly and good thermal paste applied correctly (http://www.arcticsilver.com/arctic_silver_instructions_big2.htm)? Power Supply shorting out; is something touching the case or a standoff that shouldn't? (unbolt from case and test on a piece of cardboard)

we're going to need the specs of the system, especially those of the power supply.