I have been attempting various 'fixes' on a Dell Inspiron N5010 laptop since the release of Windows-10 Build-1511. The original Win-10 installation was fine but the 1511 Update consistently failed at 40% (23% into the Drivers & Features part). Non of the 'fixes I tried actually worked and the installation continued to collapse at 40%.
Last night I was trying a new (to me) Driver Updating program from Auslogics (Driver Updater) in an attempt to fix an issue on my Advent Desktop - after purchasing the full copy of the driver updater it downloaded drivers which other driver update programs had not found before and it DID fix my issue. Consequently, I decided to try it on the Dell laptop as I wondered if it was a driver issue that was preventing the update to Build 1511.
The Auslogics Driver Updater found half a dozen drivers that Neither Driver Max nor Driver Booster 3 had found and after installing them I tried the Windows Update again. This time it ran through to completion without any problems at all. I am still uncertain as to which of the drivers was the problem but obviously Microsoft needs to take a look at their update code and to download necessary drivers when an issue is identified (or at least give a more meaningful error message than merely "The update has failed".
I hope this helps others with the same or similar problems with the 1511 update. I would be interested to hear how you go on.
Steve