Thanks FyfeFlyer, I have followed both of your suggestions but the situation remains the same. I think that there is more than meets the eye here because the computer has got very slow, the mouse only seems to work when it feels like it and a program that I have been using with no problems, suddenly won't run. This latter development is, apparently, because it has had its permissions for writing to the registry withdrawn. I haven't withdrawn them (don't know how to) and no one else uses the machine so it is all a mystery as to what is going on.

I have defragged, registry-cleaned and used Ccleaner to clear up temporary and other files, all to no avail

I have checked for viruses and other malware but didn't find anything