http://www.theanswerbank.co.uk/Technology/Computers/Question1211821.html
I have managed to resolve this problem with a little help form the folk where I bought my PC 4 years ago and from a local PC repair shop.
After repeated crashes in various start up modes I reinstalled Windows. This did not cure the problem and I was advised to run "Memtest 86". I duly did this and it immediately found errors on both memory modules. I purchased one new module and inserted it on its own. I then uninstalled Windows and did another clean install.
This had the desired effect and my PC is now running smoothly again, albeit on only one memory stick. I am trying to invoke the "lifetime guarantee" clause from the stick manufactures (Corsair) but at the moment they are prevaricating.
I just thought that everyone who answered my original question would like to know the result of my endeavours.