My computer has taken to switching itself off several times per day, around 20 times yesterday, which as you can imagine is very frustrating.

It goes completely black and unresponsive, but the lights on the keyboard flash and I have to hold the power button in for 20 seconds to switch it completely off before I can switch it back on again.

It seems to be connected to me trying to access anything which has graphics, so I can't play any games. It only used to do it with pop-cap games, but now is doing it with everything I do, such as facebook etc.

AVG scan very rarely makes it the full way through. I have a feeling it is to do with lack of memory, but I don't know why as there seems to be plenty of space left on the hard drive.

Can anyone help?

P.s. can any help be phrased for a techno idiot please. Thanks.

Lack of memory (either temporary storage - RAM or on the hard disc) is unlikely to cause this sort of problem.

As the keyboard is still responding, the computer isn't actually off. It seems that while using graphics intensive programs the display has issues, which would point towards the graphics system of your machine.

My first action at this point would be to uninstall the graphics card drivers then reinstall the drivers.

You could also try refreshing the DirectX componants, while I doubt that is the cause it would be good to rule it out.

EDIT:
Re-read your post about 'games'. Seems you are not running intensive games but browser based games right?
In which case I'd be suprised if that pushed the graphics system hard enough to cause any problems.

See if you can spot a pattern with amount of time the machine has been on.
Does it tend to happen more when you have a lot of programs/windows open?

If so, I'd lean towards something getting too hot as the system works harder it generates more heat. If an old machine it may be worth removing the side of the case and cleaning fans/cooling fins of dust that may have built up.

Thanks Dafoot.

My son installed me a new graphics card a few weeks ago in case that was the problem, but it has continued switching off in the same way.

I only play little games such as scramble etc, nothing major.

Yesterday it was switching off sometimes before finishing starting up from the time before, sometimes only after an hour or two.

I do think temperature may have something to do with it, and I will dust it - very likely about time :P

I have dusted it all out now, it was pretty dusty inside the fans.

What sort of temperature should a computer be running at? Mine has a thermometer on the front, and at the moment it is at 103.4f and seems to be quite happy. I have seen it go up to 129 before now.

I am just about to go and play on a game to see if it survives - wish me luck :)

104 is about 40C which is well within normal operating temp range.

129 = ~54C which is warm but shouldn't be too hot.

Are the temperatures you give from the case temp or the CPU temp?

If 54C is the case temp, I would worry.
If that is a CPU core temp, it's warm but wouldn't think it would cause failures. I guess it depends on tolerances for your processor.

If you google for the tech spec for the processor I'm sure you'll find the recommended operating temp. range.

I'd also pull the power supply out and give that a good declogging, could be that that's on its way out. A can of compressed air would be handy for cleaning the PC out properly, but try not to damage the fans whilst cleaning them with it.

Well, touch wood, it hasn't switched itself off since I took the side off and dusted it all out :)

The fans were pretty full of dust to be fair, so maybe that is all it was.

There is a blue light stick which makes it glow a pretty colour through the transparent casing, which had come loose and may have been obstructing one of the fans (there seem to be loads!), so I have stuck that back in place too.

Anyway, so far so good, now I might try playing a popcap game as the ultimate test :)