Windows Update stopped working. "Jay in camborne cornwall" stated on Microsoft technet that the problem is that the Administrator account is disabled, but it must be enabled for Windows Update to work.
However, when you go to the User Accounts control panel and click on "Manage another account", you don't see the Administrator account! An article on howtogeek.com shows how to enable the account via the command line.
After following these instructions, Windows Update started working!