PC won't boot past POST?

Hey,
Im having an issue with my vista machine no longer getting past POST. It starts and then shuts down in 3 seconds. According to the POST codes on the mobo display, as far as i can tell it gets to C3 just as it shuts down. The manual refers this to memory, but im not sure as it could be C4 its actually halting on. Just to clarify I am getting nothing on screen at this point.
Things I have tried:

All the usual resets, disconnect from mains etc.
Checked all connections thoroughly
Cleared CMOS
Then tried removing unessecary hardware (leaving 1 graphics card, primary sata and 1 stick of memory)

..Still no difference, shuts down after 3 seconds.

My spec:
(Nothing currently Overclocked)

Intel Q6600
BFG NForce 680i SLI Motherboard
4GB (4X1GB Sticks) Corsair 8500 Dominator DDR2
X2 BFG 8800GTX 768MB SLI
Creative XFI Sound Card
CoolerMaster 1000W PSU
WD RAPTOR-X 150GB
WD CAVIAR 500GB
Vista Home 32Bit SP1

The system was working fine until this morning when it would not boot. Before that it was only running media player and 1 download. No problems or crashes whatsover, or indications of anything before. The only other thing I have noticed is that my HDD LED and Power LED on the front panel of my case are on constant. This only did this once before when the computer went in hibernate mode, however I disabled this feature a long time ago with no problems since.

Any help greatly appreciated!

I'm thinking PSU problems if the problem is the same with every memory stick individually in each slot.

Other possibility is graphics card - you could try rebooting without that to see if it stays on more than 3 seconds. What does code C4 indicate?

Only real way to test it it is by substitution. If it isn't the PSU, then the mobo would be the next thing I'd look at.

Thanks very much for the swift reply.

Im trying to get hold of a spare psu to try asap, and will post back. I believe C3 is Extend Memory, and C4 is Special Display, according to the POST Codes in the motherboard manual.
I will also try removing both graphics cards, as up until now I have always left one in.

Thanks again.

Ok to update...

I got hold of a new psu today, tried that but no difference. It still shutdown after 3 seconds.
So I decided to play around with the memory sticks, as the Bios codes were pointing towards that. I left the one stick in the nearest slot to the cpu and this made no difference, still the same problem.
Then I removed all sticks and started with a different stick in the furthest slot away from the cpu.

This time we had activity! It hung on a few codes, before finally coming to rest on code 7F. This code refers to POST ERROR CHECK: Display and wait for user intervention.
Through all the testing etc I had removed my monitor, so I plugged it back in and hey presto I was greated with the POST screen and the following:

Floppy drive fail (40)
Checksum error - bad checksum (defaults loaded)
Press F1 to continue, DEL for setup.

Great, so I jumped into setup, removed the default floppy boot etc because I dont have one, saved and exit, leaving all other settings at default. It rebooted fine and got into Windows no problem.
So to quit rambling, the bottom line is, it is now up and running fine, back with all sticks of memory added (4x1gb sticks). I simply reinstalled them one at a time from right to left.
I am yet to put the 2nd graphics card back in, but at this point it seems fine. Yet Im completely stumped as to what exactly happened or caused this, and why its fine now.

If anyone has any ideas, I would love to hear them as Im fresh out:confused:
Thanks

computers are stupid. sometimes mine wont boot if i've left my usb pendrive plugged in...

Haha, funny you should say that Streetster, but I had a problem back a few months ago regarding my stick drive screwing up usb settings or something. I had to restore Vista from a point a few days before, and plug the drive back in....very strange.

Well so far Im typing this on the 'problem' machine and its ok (touch wood). Also ran some prime95 and the memory is holding up well.

I'd point at the motherboard. Some of the AMD NF4 boards would mysteriously fail to boot. The accepted remedy was to unplug the power and take the CMOS battery out and leave overnight. This usually worked.

Did you upgrade the BIOS recently? If not let's hope it was a one-off. Just one thought - you might want to check that you don't have a problem with the chipset fan.