Recently whilst playing LFS my PC will just turn off. I've played other games and this doesn't happen. It happened twice yesterday while on an STCC server on the last lap whilst in the lead. Then about half an hour ago it happened while on a fresh install of LFS running tweak on singleplayer.

My GFX settings are on no AA and no AF so it shouldn't be the GFX card killing it.
I've done virus, ad/mal/spyware scans and it's found nothing.

At a rough guess I think it's PSU related as i'm not able to switch the PC back on again for about 5 minutes (after cooling down I guess).

Does anyone know what could be causing this to happen?

It is your CPU that might be overheating and calling thermal shutdown to prevent core meltdowning.

Here is a free nice program to check the temps and other stuff of your PC.
Everest Home Edition 2.20 http://www.majorgeeks.com/download4181.html

Install it then run it and go to the Computer>Sensor node to check the temps.
Then without closing it launch LFS (use Shift F4 to go to window mode if you like) and monitor the temperatures.

You should check temperatures, voltages and fans with an utility while using heavy loads. If the problem didn't happen before, try opening your case, check that all fans are working and that the airflow is sufficient. Clean your pc if needed (if you use compressed air stop fans gently while you do it). There are some benchmarks that could help. I would use a gradual approach testing the cpu at full load first while monitoring the system, then the gpu, and then all. Most monitoring utilities that come with motherboards allow you to have an adjustable acoustic alarm for parameters that are going wrong, I'd try to take advantage of that. Check the event viewer too, there might be some hints.

Edit: I'd start with Prime95 as testing utility.

Thanks for the help so far.

I checked my CPU, heatsink and fan and they were quite dirty so I cleaned them up a bit hoping it would solve it a bit but no luck. I also cleaned the rest of the inside just incase.

I downloaded and installed the Everest program and ha dit running whilst playing LFS for a while and the CPU temperature went from idle 40c upto 54c and the PC didn't die.

If you want to see any of the info that Everest displays that could help me then I will post them, but i'm not sure what you'd be looking for.

What is the recommended running temperatures and what should it go upto 'til it shuts itself down?

I also tried that Prime95 thing and couldn't get my head around it so stuck with Everest.

I'm finding this problem very frustrating as it has never happened before and i've owned this PC for atleast 4 years only upgrading the RAM and GFX card. It only started happening a few days ago after moving bedrooms.

Thanks again people.

Once u locate the problem, u could install a Nismo intercooler in the front of your pc, it should keep everything cool

The cpu temperature is fine, no problem there.

There is a problem if you go above 70 and even then the Intel CPUs start throttling (degrade the performance) and the PC auto shuts down at 80 or something I think.

But there also might be a threshold about the auto shutdown temps in your BIOS settings. Check this out also just in case it is too low.

What about the GPU and motherbord chipset temps? Are they also low? A screenie of the sensors page might be handy here.

And a final thought is to unplug and replug all your internal cables just in case something moved (slightly unplugged) during this movement you said you made.

Only motherboard, CPU and HDD temperatures were displayed. I looked on the GFX card section and couldn't find the temperature.

Is there any other way to find it out?

The screen should be something like this

Can you post yours?

All I get is what's on the screenshot.

My GFX card is a nVidia FX5200 if that helps?

I'm not sure about older nVidia cards, but at least GF6 series have thermal sensor. In newer driver versions(83.xx and higher IIRC) there is no problem to read the temp from Control Panel(Start>Control Panel>Display>Settings>Advanced>nVidia FX5200>Temprature Monitor).

No temperature thing from looking where you said. I'm using 84.43 drivers.

You still might want to upgrade, but I'm afraid that there is no temp sensor on FX series cards. Then you should at least clean the GPU fan and see if it helps.

Another thing might be your PSU, you've said that you have your PC. PSU power degrades how PSU is getting older. Borrowing some 100% OK PSU enough powerful to handle your system adn give it a check is the only idea I can make up ATM.

After all it was my PSU. It died on Saturday.

Since that PC was getting a bit old, I've ordered a brand spanking new one which should be here this week or next.

Thanks to those who tried to help.