(Note to my regular readers: The only reason I wrote this post is because there's not much help for this situation to be found on Google. Don't worry, I won't be blogging about weird error codes from now on.)
Today I wiped out my system and did a clean install of Vista.
While it was downloading updates I kept getting the above error code for two updates. I googled it and found nothing but a bunch of posts that more or less said "what does code 80071a90 mean?"
Well, I can't tell you what it means, but I can tell you how I got around it.
On a shot-in-the-dark guess, I set Windows Update to just install one of the two updates. As the updates were most of a year apart, I set the older one to go first.
It installed fine.
I restarted and installed the other one.
It installed fine.
So I guess the code means "these updates don't play well together, install them separately."
(if anybody wants to know I was installing Ultimate x64 on a homebrew machine)
UPDATE 8/17/12: Amazingly, this is by far the most-visited post on the blog and still gets comments every once in a while. Because of time constraints I have now, I've disabled comments on this post since I don't really have time to moderate them anymore.