I solved my (0x80070643) problem...
So after spending 80 minutes on a support call with Microsoft, where at the
end of the call the answer was "you need to back up your files and reinstall
Vista" I thought there must be a better (and less time consuming) answer.
So once again I started at the beginning (of all the suggested fixes) and
the first thing I tried was manually running the mpas-fe.exe program that
gets downloaded to the C:\Windows\SoftwareDistribution\Download\Install
directory.
This time, I happened to notice that during file extraction, it was
extracting the files to my "E" drive. I am running Vista Ultimate on my
MacBook Pro in the second partition on my hard drive created by Apple's
BootCamp utility. I use a utility in Windows Vista called MacDrive that
mounts my Macintosh partition (as drive "E") so that I can access my Mac
documents while I'm booted into Windows.
When I'm booted into the Windows Vista patition, my Vista drive is seen as
my "C" drive (as you would expect), and the Mac volume just looks like
another drive. For some reason (not sure whether it is Vista or MacDrive
that causes it), Windows Update (and Windows Defender) extract all of their
temp installation files to my "E" drive instead of my "C" drive. All of the
Windows Updates always install just fine, but for some reason Windows
Defender doesn't like it when the temp files get extracted to something other
than the "C' drive.
So I temporarily disabled MacDrive and forced Windows Defender (via the Help
menu) to update the definitions. Lo and behold it worked just great! Now
defender is up to date and I don't get the 0x80070643 error message any more.
So... In the end it was MacDrive for me--and the fact that I had a second
drive mounted as "E". For those of you also having this problem, check to
see if you have another drive mounted other than "C". That might be part of
your problem.
Also, there may be a way (but I couldn't figure it out) to manually extract
the files from mpas-fe.exe to your "C" drive using a command line switch so
that the files can run from there, perhaps that might also fix the issue.