Windows Update ffice 2007 fails

This one is confusing the heck out of me!

I'm running Vista Ultimate, and Office 2007 Ultimate.

When I did a Windows
Update I was told of four security updates to Office. They download
properly but do not install: when I try to install them I get a consistent
error on each of them: Error 91A.

I can find no such error in Microsoft's Knowledgebase. No solution in the
other Microsoft.public newsgroups to date.

I tried updating in Safe Mode and get the same error message.

What should I be searching on to identify and alleviate this problem? Any
idea?

Thanks!

Ross