Thanks Giridhar J -
The problem seems to be resolved. After a couple of hours at the "67% complete - don't turn off your computer" screen I finally decided just to take my chances and power off. An hour or so later when I turned the computer back on it gave me the option to start in safe mode or just boot to Windows normally. I selected to start at regular Windows. It next said to wait while Windows was being prepared - which went fine. No more hang ups and everything is working OK. I checked the status of Windows updates as you suggested and saw that a number of updates did install successfully with no failures. Whatever the problem was it seems to have fixed itself. Thanks for your response.
Waldi