ManfredS's fix didn't work for me. Well, it got me another step along the road, although it may turn out to have been a sidestep more than anything else.
I now get the message that "The Windows 7 Service Pack Installation cannot proceed until your computer is restarted. Please restart your computer and try again."
It doesn't matter how many time I reboot, it doesn't help. *sigh*
Have also tried JStar333's similar fix detailed here but there was no difference.
Going to see if there's something in the registry about the pending reboot and, if not, I'll try a Safe mode install.