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I found the version numbers of the files that were updated, here: http://support.microsoft.com/kb/975558. The guilty file was version 11.xx instead of 10.0...

I had the same issue on a new installation of windows XP itegrated with SP3 and updates to Dec 2010. I found one file, mp4sdmod.dll, which had a higher version number than it should have. I renamed that file and copied the one from the windows update folder C:\WINXP\SoftwareDistribution\Download\bc81666f3868f34642e3f5adbc2719f9\wm10 When I then ran the update it installed straight away. Perhaps just deleting the old file and then running the update will do the trick too.