New System won't boot !!! (CPU sespected) ( Core 2 Quad ) ( Asus P5k)

My new system won't boot up . It turns on for a second or so, then shuts down. I tried different startup configs of hardware, finally the system powered on and stayed on after I removed the CPU ( Core 2 Quad). Everything else seems to be working.

My motherboard is Asus P5K Deluxe. Someone told me that it could still be the MoBo. I put the CPU in Dell XPS system, but didn't work. I also put the pentium D from XPS in the new system, the system turned on for a second, then shut down, then turn on itself again and stayed on with all the fans, and leds working. But no display.

Power Supply seems to be in good shape, With all the fans coming on. GPU fan comes on, Optical drives work. Don't know anything about harddrive, because plugging or unplugging it didn't matter.

My conclusion is that it is the CPU, but Motherboard also is suspected. Please help me figure out this problem. I'm getting a cheap 775 socket motherboard from local store to test the CPU. If the CPU works then I guess I will RMA the MoBo.