I have been searching for quite some time now to no avail on how to either remove or re-install failed updates. I have many PCs that don't like the recent Update for Root Certificates for Windows XP [April 2012] (KB931125)
Here is what I see in the windowsupdate.log
80070643 AutomaticUpdates Failure Content Install Installation Failure: Windows failed to install the following update with error 0x80070643: Update for Root Certificates for Windows XP [April 2012] (KB931125).