Posting this here because I'll probably encounter it again, and it was a bit thorny.

XP Update KB2686509 wouldn't install, no matter what I tried. Most of the online help suggested problems with a faultykeyboard.log file, which I didn't have. Finally, I ended up backing up, and then deleting, this entire reg key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layouts

Then I rebooted, and ran the hotfix from the downloadable-file version, and it worked.