Did you install after turning off your AV?

Here's a bit of an update,TaurArian.   Sorry to take so long, but I have had
further computer problems which have kept me busy.

I looked through all your advice, and the most relevant solution seemed to
be your first suggestion:

I went through KB949377 carefully.   I had already tried Methods 1 & 2
several times, so I went to Method 3.   I downloaded and installed
Subinacl.exe, then ran reset.cmd ("Run as Administrator" was not an option).  
I then installed SP3 from the downloaded file (323 MB), and it seemed to
proceed OK.   After about an hour the install completed and was asked to
restart.   As Windows was loading I got the following message:
"ACMON ERROR
Failed to prepare Critical file!
Chameleon Engine out of service!"
Windows then completed loading;  however, there was no Task Bar or Start
button.   When I opened Windows Security there was a message to the effect
that Windows Security was disabled.   With these problems, and perhaps many
more that weren't apparent, I decided not to proceed with SP3 and restored an
Acronis True Image of Drive C.   I have been using my laptop without SP3
since then, with no further problems.

However, that was not the end of my problems.   While my laptop was being
restored to pre-SP3, I went to my main PC to post a response here, but it was
then running so slowly it was virtually unuseable.   I had installed SP3 the
day before with no problems, and it seemed to be running better than before
so I was surprised that it was now misbehaving.   To cut a long story short,
after a couple of days of troubleshooting, changing hardware parts, and
consulting a friend who was having a similar problem, we discovered that the
cause in this case was CA Anti-virus.   When my friend and I uninstalled CA
and did a Registry and file cleanup, our computers are running better than
ever.  I hope this helps someone.