Near catastrophic update!
I just about got this installed without losing my machine. Twice I tried and twice the computer hung on a restart at "Updating windows 51%". I left it for six hours and nothing happened and I had to pull the power cord to turn it off. After that I had to rollback and repair.
To get the update to work I had to run the Microsoft Windows Update Repair Tool (downloaded from their website) and then run the update a third time for it to work.
Huge sigh of relief and glad I know what I am doing or I would have been shafted (like many a poor user out there I am sure).