I have also taken the various cards and memory out of the motherboard and re-installed them but that doesn't rule out a fault.
I went through the advice in the link you kindly proferred, but found none of the suspect files in the drivers folder.
After another crash, windows alerted me to a Spyware virus, Spyware.apropos.C, and directed me to Windows Live OneCare, which I have just finished running.
It found a problem, and rather unhelpfully finished saying that one problem had not been fixed.
But for now all seems ok, fingers crossed.
Thanks again for all your help, hope your keyboard's ok now