I recently faced the problem that I could not install KB968930: Update for Windows XP. Installation failed.
This update contains the Windows Management Framework Core.
This installation failure only occured on a desktop computer, which had Windows XP Sp3 workstation and the Exchange 2007 Management Console installed.
If you face the same problem and you want to install this update with Automatic Updates, you should first remove the Windows XP update KB926139 (English version) or KB926140 (localized installation version).
After a succesful uninstall of KB926139, installation of KB968930 will complete succesfully.
I’ve never found out why it is like this, but it works. :)
I’ve also found out that you can probably install KB968930 manually, without uninstalling KB926140 first!
You can download KB968930 here: http://support.microsoft.com/kb/968930