Peter Bromberg's UnBlog: KB929729 Windows Update Failure - An Easy FIX
In case anyone else runs into this... I had just upgraded a machine from XP to Vista and the .NET 1.1 SP hotfix problem was waiting for me at the end of the upgrade process. Mr. Bromberg's post of these steps fixed my problem. :-) This doesn't sound like it's a common problem, so it was a relief to find the steps to fix it so well written.