I have an e-mail into the good folks at Microsoft about this issue, but while I wait for them to respond, I thought I'd ask here.
After a significant upgrade to my homebuilt, I had to repair and reactivate Windows XP SP2 from my OEM CD. No problems there and everything seems to be working fine with one exception: windows updates. That little yellow shield sits in my tray with an update for Windows Installer 3.1 (oh the irony), and every time I try to install it, it fails. I don't get any error messages (unless I just don't know where to look for them); it just fails and tells me it failed. And there that yellow shield sits. Every time I shut down, it tries again and fails to install.
The other bit of odd is that, when I go online to the updates area of microsoft.com, there's nothing in my history. No updates listed as being successful, and none as having failed. Plainly something is amiss.
If anybody has experienced this and was able to solve, please share. Thanks for any help.
Thanks for the comments. This turned out to be no big deal; the folks at Microsoft e-mailed me with instructions on what I should delete on my machine and a zipped .bat file I could run afterwards to set myself up again. I'm updating painlessly once more.