Shay, Thank you. By the time I read your email (Sun lunchtime) I had solved the problem in what I am sure is a most inelegant and clumsy fashion. I am nonetheless grateful for your suggestions, which have been filed away in case the problem reoccurs.
The disk was a DVD of Ice Age which my 7 year old had borrowed from the local library. Half way through it froze, and then the MacBook flashed up an error message saying the disk was damaged. The machine would not restart or shut down or log out the current user, the eject button in DVD player would not work, the power button did not work. Closing the lid put it to sleep, but on waking up everything was still the same. Folders and applications could still be opened, and that proved the path to the eventual solution (Activity Monitor). Should anything similar happen again I have your list of possible fixes. But I do have one further question - is there some way in OS X (10.4.8) of forcing a restart, as the keyboard combination command-option-powerbutton does under OS 9?? Thanks again, Bill Chesnutt
