I have a PC with Windows XP loaded. Whilst using the PC last weekend, it shut down without notice, I was in the middle of typing in Word and the machine powered down. I switched it back on, it went through the screen to access the bios, memory check etc, then asked if I wanted Windows to start in sade mode, or normally etc. Before I had chance to make a selection, the PC shut down, I did not see any error message appear. I tried again, and the same happened. Third time, I interecepted using F5, and asked it to load Windows with prompt, it got so far and switched off again. Fourth time, I went into the BIOS and before I could get beyond a selection, the machine shut down again. It appears that the PC is powering up for a certain length of time only, then shutting down. I have had problems with the fan, although this was a few months ago, and this was resolved.

I would be grateful for any advice. Thanks.

If you leave the machine powered off for an hour or so and then try to power the machine on, does it get farther along in the startup process than it does when you immediately try to start it up after a shut down?

This has all the signs of overheating. You may want to make sure that the fan problems that you had a few months back aren't happening again. Open up your system case and watch the fan on the CPU as the system starts up. Notice if the fan spins or not (it should spin very fast).

A good test to do to diagnose a heat problem is to open up the case (take off both side panels if possible) and point a very powerful fan directly into the case (make sure that you are blowing the fan towards the side that has the CPU on it). If doing this allows your computer to run properly (or allows the computer to run longer than usual), then you definately have a heat dissipation issue.