Thanks for the reply madmonkey.
Has any new hardware been installed on the PC recently?
No, none. The error has occured since day 1 of having the PC.
Is this problem random, or does it occur when performing a specific task(s)?
Pretty random I think. Main times it has happened is when watching movies, downloading files of internet (when this is done PC is in lock mode - windows button + L). Decrypting DVD's, virus scans. Yeah pretty random. Even if computer is just locked in standby mode, with nothing running except for the usual virus scanner etc.
Do you have surge protection on your PC?
I have a Surge Master adaptor plug that it is plugged into, rather than directly into the mains supply.
One of these:
Belkin Maximum Series 8-Socket SurgeMaster with Tel/Fax/Modem & AV Protection 2M Cable (£175,000 Warranty) - Ebuyer
Have you been overclocking your PC at all?
No. Overclocking is currently set to AUTO in the BIOS, which according to the manual is off.
Can you open your PC, and look at the capacitors on your motherboard, check each one to see if any are bloated or showing signs of leakage. Leakage will show as a black substance at the base of the motherboard or on the top of the capacitor itself, will appear dark orange, almost like rust.
Not too technically minded on that part. Are the circled red parts the capacitors or the circled blue parts?

Also check that all the fans are spinning when you switch your PC on, and press down gently on each of the cards to make sure that they are firmly connected.
Only got 2 fans. One on the front of the case and one on the
CPU, both working fine.
RAM is all locked into place and I only have a nVidia 6200 graphics card, which is in snuggly.
Anything further you need to know please ask. Thanks again for the advise.
Pete