As mentioned above, creating a new (admin) user did not cure the problem.

It also created a new one.

Until I created the second admin user, I had only one user and there was no select user screen. When I booted up just now, was away from my computer and after a few minutes came to the user selection screen and found that the lack of click-responsiveness also applies to the select user screen, not only the taskbar. Perhaps it applies to all OS areas of the screen. (I seem to be able to click on anything in any non OS, program window area).

It took me some time to select one of the two users. I had better delete the new user otherwise I may not be to enter windows XP.

Tim