Having searched for a driver update for my video adapter, and being told there was no later one, that did not solve the problem. Then the very next day I went to Windows Update and it offered a driver update which was dated July 2003! This was the first time it appeared in spite of regular visits to the update site. I loaded it and it solved the problem. Thus the cause appears to have been the need for an updated driver. I'm still baffled why my regular checks did not show a later driver available.
