Computer keeps rebooting, please help

[Aeria]

I was siting at my computer playing Eve Online earlier, when all of a sudden my computer rebooted.
It ran as normal and got to the "Normal or Safe Mode" screen. I chose normal , and it rebooted while the loading bar was scrolling at the bottom of the screen. Next time, I tried safe mode and it didn't fare any better, rebooting with some text on the screen, and before showing the cursor or windows logo.

After this I turned off the computer and left it for 20 minutes, then tried booting again.
After several tries: Normal mode was the same as before, while safe mode got to varying positions before rebooting, just getting to the windows desktop before rebooting in the best case.

I tried running Memtest from CD -- over several tries, this resulted in reboot by test 3 at the latest, without displaying any errors.

My specs: Vista 64 Business, e6400, 2*2GB OCZ PC6400, Gigabyte GA-965P DS3 (rev. 1), 8800gts 512MB, Samsung Spinpoint HD501LJ, X-Fi Xtreme Gamer, Seasonic S12 430W PSU, DVD-RW drive.

The BIOS has not been reset and detects the cpu, memory, gpu and drives as normal.
I haven't moved the computer or added/removed any hardware recently, and a quick look inside revealed no loose cables or burning smells.

Any ideas?

[TAKTAK]

try booting with one stick of RAM

if that works then bump up VDIMM a little bit as OCZ do like their juice

[Aeria]

Well, I tried it with just one stick at a time, neither got any further into windows. Next I tried using my spare 256mb pc5400 stick (on its own) , this didn't work either. In all three cases Memtest ran for longer than before, but still ended up rebooting. By the way, nothing is overclocked.

[TAKTAK]

try booting with the barebones in the system...

[Aeria]

Ok, I tried it:
Without the sound card - no improvement.
Without the sound card and with the 8800 replaced by a Voodoo 4 pci - memtest still reboots, manages to get all the way to a usable desktop in safe mode, reboots after about 5 seconds.