|Thankyou Ed and Seawatch.|
I visited Bleeping Computer as you suggested and there does not seem to be anything unusual in my Start Menu or Startup.
My mouse is a Microsoft cordless job (as is the keyboard) and it seems to be operating OK. I replaced the batteries in both units just in case.
As I said the problem is infrequent and as a result it is hard to pinpoint.
Thank you for your replies
The problem with Socialism is that eventually you run out of other people's money!