Help! Rig turns on for a moment, then turns off...

Try booting with just one stick of memory.

It sounds like your memory may not be seated correctly. I've had a similar problem on a laptop, so you may wanna check that out. Good luck.

Possible reasons for this problem:

If you are over-clocking, this is the first suspect. Restore to defaults by clearing the BIOS. Otherwise:

1) Memory
- Verify memory voltage is compatible with your motherboard.
- Verify memory is installed properly for your configuration.
- Try booting with each module installed individually. You may find a bad one.
- If you do boot, try running MemTest86+
- Try other RAM if you have it handy.

2) Processor Overheating
- Verify CPU fan is plugged in and operating.
- Reseat heatsink (you did this, try using the stock heatsink).

3) Loose wire
- Verify all power cables are installed snuggly.
- Verify the 4-pin CPU power connector is installed properly.

4) Motherboard installation
- Verify the motherboard is not being shorted somewhere on the case. A common example would be if you failed to install the motherboard stand-offs and screwed the motherboard directly to the case.

5) Faulty PC case power switch
- Disconnect switch from motherboard. Power on PC by briefly shorting the two pins on the motherboard with the tip of a screwdriver.

6) Faulty Powersupply
- Try a different PSU if you have one handy.
- Try your power supply in a different PC
- Use a multimeter or PSU tester to verify it's operating normally

7) Clear the BIOS, you never know.

8) Faulty/damaged CPU or motherboard
- Difficult to verify unless you have alternate parts to try.

9) Last resort
- Rebuild PC outside of the case on a static-free, non-metallic surface.

Tried booting with each stick of RAM seperatly, didn't work.

Not overclocking yet- haven't even gotten the thing to start up once to get in to the BIOS.

We rebuilt the entire thing, so we did reseat the heatsink. Going to try using the stock heatsink after we get something suitable to clean the old thermal paste off the CPU. This also wipes out the loose wire thing.

What do you mean as far as the motherboard stand-offs?

You might need an old processor...or maybe the guy at the comp store can do. Just take the bare board in and ask him to try a c2d E6400 in it.
You'll see the bios rev. as soon as it starts.

A lot of work, and you may have done most of it:

http://www.tomswiki.com/page/Troub [...] +New+Build

Keep in mind that I wrote this before compatibility problems became so (relatively) common.

We rebuilt the entire thing, so we did reseat the heatsink. Going to try using the stock heatsink after we get something suitable to clean the old thermal paste off the CPU. This also wipes out the loose wire thing.

Isopropyl alcohol and a piece of a coffee filter or lint-free rag work nicely for this.

Check the Toms sticky for trouble shooting a system, where they recommend an out of case build.
Then build from there, ensure all connections are made and proper.

Depending upon just how long it runs before shutting down your bios may tell you what's wrong using "beep codes". Have you heard any beeping from the PC speaker before it shuts down? I've recently built two 775 boxes and had no trouble with the OEM heatsink/fan installation. If you're having problems there I suspect the processor is overheating and shutting the system down. There are many things that will prevent boot up but few that will cause a shut down.