Desktop Building Problems, need help.

[Dark Silver]

Since Ace decided to goto sleep....lazy bastard...

Ok, so I decided to rebuild my desktop computer, since it was older than dirt (pre-2000). I got a new motherboard, processor, RAM...pretty much the only thing original on the thing is the case.

So here's the setup, then I'll give you the problem.

ATX Case
PSU: 500w Antec
Processor: Pentium D 805 Smithfield Dual Core 2.66 ghtz (LGA 775)
Motherboard: ASUS P5N32-SLI Deluxe
RAM: 1GB Giel DDR2 667
Video Card: BFG Technologies Nvidia 7600GS 256MB DDR2
HDD: 320GB Seagate SATA
DVD-RW drive with Lightscribe

Pretty much a huge step up from the previous system...which was a 600 Duron...

ahem.

Anyway, the problem now.

I have put the computer together, first time, I had not seated the motherboard properly (error on my part) and could not get even the POST, and after a few minutes, the computer would shut itself down.

I removed the motherboard, reseated, reattached the drives, (at Ace's insistance since I didn't seat the mobo properly the first time) and now I'm getting it to start up, it will begin the POST, then the computer will shut itself down. the more I try to restart to get it beyond the post, the sooner it will shut itself down.

Now, since the computer is working, getting to the post, I'm ruling out motherboard damage....500w power supply should mean it has the juice to run the beast without a problem.......or am i wrong on that assumption?

Any assistance you lot can provide, I'd be greatful for.

[Batman]

Define 'shut itself down', please. Does it go all the way down to where even the PSU is no longer running, or do the PSU/fans on various cards/ case fans/drives continue to draw power but nothing else happens?
Personally I AM inclined to rule out insufficient power (500W should be plenty) but by and by disconnecting additional power consumers might help determining when, exactly, the problem occurs and what causes it (where exactly in the POST routine does it go down?).
If it CONSISTENTLY shuts down earlier the more you try it might be a thermal problem. The longer it runs, the warmer it gets.
And on what base are you ruling out a MB problem? It could be anything from a thermal problem that doesn't cut in until temperature X to a routine in the BIOS that isn't activated until step Y in the POST to a defunct slot that isn't queried until step Z etc.
I don't know what you have tried so far but my advice would be
a)disconnect EVERYTHING but the graphics card (unless onboard) and see what happens.
If a) runs without problems, start attaching other components, make note of when the problem occurs.
If not, disconnect/disable graphics card (if possible), refer to board manual what (if any) beep code on startup means.
Just for starters.