|Well, no one offered any suggestions so I had to sort it on my own. |
In the end I pressed F11 just after a restart and this took me into HP "System Recovery" (The computer is an HP Pavilion laptop). Then I followed instructions to reinstall Vista from the original factory settings. This needed no disks (I think it was using a recovery partition on the laptop) and was really easy to do although it took a while, and I obviously had to backup files first. Then doing all the updates took ages.
But now it works brilliantly, much faster again and with none of the problems it had before.