Just to bring this one to an end, it turns out that my computer is sick with memory problems. The engineer called with 2 new memory sticks, but after taking out the old ones, and inserting the new, the computer would not then boot up. He tried them in different combinations with the old but again no luck. It has now been returned to the manufacturers, but how odd is all that. The new memory had exactly the same code number as the existing.
Thanks for your replies, but I feel that I have some egg on my face by blaming Vista, it is proof that we should not believe everything we read on the web.