I had a sleep for an hour and the computer works fine now.
I switched the computer on again and it was the same problem, so I switched off at the mains point again restarted and then I got the message saying that the computer could not start and suggested LAUNCH STARTUP REPAIR which I chose and that fixed the problem. I agree with your comments totally about the computer compressing and defragmenting whilst inactive. How could I have chosen LAUNCH STARTUP repair earlier? Is there a way of getting to this before Vista decides itself that it has a start up problem?
Thanks for the advice to save my data to another device.
The original Vista set of 3 installation disks were lost during a previous house move and tried getting these last year from Fujitsu Siemens but they said they couldn't help me :-(
Which external drive would you recommend as a backup? a 2nd New Hard Drive or something else?