Will have a look tonight after work and see how I get on...will let you know.

Thanks all.

This same thing happened to me once, but I was playing Silent Hunter 3.
It turned out to be the CPU that died on me, which is rare specially on a non-overclocked rig.

well I just sent it back to the retailer and got a replacement.

have you changed any BIOS settings? the only time I've had something similar is when I've messed with the BIOS for OC'ing and set wrong parameters. You might reset it anyways just to rule it out (either take the battery out or short the pins (should be in the manual).

If it's not that then I'm guessing it's a heat/power issue like the others suggested

I never OC and I haven't touched the BIOS in ages. Tried another PSU and the same thing happened so it's not that. Took the HS/fan off and the paste that was on has dried up and is pretty non-existant so it may be that, either overheating or as Spede mentioned it's damaged the CPU.

It's 2 years old so I doubt they would replace mine. I am thinking of getting a whole new rig actually as I couldn't get the HS/fan back on and thought I had cracked the mobo with trying to push it back on - I hate the Intel HS/fan units! Just need to see if I can get the funds!

The date/time was correct? Not the OS, in the BIOS screen.
When that battery goes all kinds of odd things can happen since the system clock isn't keeping time.

It starts for 2-3 seconds and restarts itself...doesn't get anywhere near the BIOS screen!

Maybe time to pull as many cards, drives, and memory modules as possible and see if you get a boot with a minimal build? Even with no HDD or OS getting to the BIOS would be a hint.

PS... One of my mobo failures had undeniable traits, such as the PS2 sockets not working anymore. Another was very subtle, the mobo would return errors with the memory module in the second slot. Any module would error there after hours of Memtest. The contact joints between the socket and board had failed, somehow.

I did a minimal build test, just mobo, graphics card, RAM and CPU connected and still the same. I am going to use as an excuse to buy another and try and fix afterwards and use as a backup machine so still need to work out what it is. I have always built my own PCs and always sorted out the problems but this has me stumped....

Yeah, that's bad. LOL
The yank everything plan sounds right to me too, can you pull that graphics card too? Do you have onboard video?
I wouldn't think it's a heat issue that fast, if it does it cold, but does sound like a hardware issue and something is just malfunctioning.
How many sticks of memory do you have? Can you scale back to 1 stick, test, then yank it and put in another stick etc?

If the "yank everything" trick can't get you to the BIOS or to a "no operating system found" message, I would suspect the motherboard has gone south for the winter.