New build - turns off after 0.5 seconds

[Festisio]

Am building my new system. Installed all the components. When I turn it on the CPU fan starts spinning and you hear it beginning to boot up but about half a second later (if not sooner) it turns itself back off. It beeps once.

What could I have done wrong, that is detected so early on in the boot phase?

The heatsink is attached properly:

Components:

2 x OCZ 4GB Kit (2x2GB) 800MHz/PC2-6400 Memory Platinum Performance CL5 (5-4-4-18)
1 x Samsung HD753LJ 750GB Hard Drive SATAII 7200rpm *32MB Cache* - OEM
1 x Antec Nine Hundred - Gaming Case with 200mm Top Fan - No PSU
1 x ASUS P5K AiLifestyle Series iP35 Socket 775 eSATA 8channel Audio ATX Motherboard
1 x Corsair TX 750W PSU - 120mm Fan, 80+% Efficiency, Single +12V Rail
1 x BFG 9800 GX2 Dual GPU 1GB DDR3 Dual DVI HDMI PCI-E Graphics Card
1 x Intel Core 2 Quad Q6600 G0 Stepping (2.4GHz 1066MHz) Socket 775 L2 8MB Cache (2x4MB (4MB per core pair) Retail Boxed Processor
1 x Optiarc AD-7200S 20X DVD±RW/DL/RAM Internal SATA Bare Black Drive - OEM

Thanks in advance

 

[Onus]

Check the power switch to see if it is shorted. Disconnect it from the motherboard and see what happens. Power-on the system by shorting the pin headers with a screwdriver or paperclip.

I had a case on which the factory drone who assembed it had run a screw through the power-switch cord, causing this problem.

 

[olimd]

Check the RAM, also are the extra cables connected to the graphics card?
Also check that the graphics card is compatible with the motherboard.
Is the Processor power attached?

 

[groo]

might want to check PSU voltages. I think there are a couple of pins to jump to fire it up when not attached to the computer

 

[roadrunner197069]

Try with one stick of ram and reset the cmos after you pull one stick out. When it boots change the ram voltage and timings to specs. The ram might even need 2.1v for all 4G.

 

[level101]

Sounds like the RAM is not installed right. Take out the the RAM, put it back in, then boot computer.