So you may or may not have heard that Defender is the default anti-virus client on Windows 10.
In previous OS versions the anti-virus client was replaced by System Center Endpoint Protection (SCEP) software when it was managed by SCCM.
You will need to remove any group policies that may have been configured to disable Windows Defender on domain machines.
I created a separate Windows 10 SCEP policy and deployed it against my collection of Windows 10 machines, whereupon I applied a SCEP 2012 standard desktop policy, modified to include Outlook 2010/2013 exclusions.
(Uncheck those 2 boxes so the future will be well with this.) To fix it: Type in the Start menu then press Enter. Then right click on this and choose "Run as Administrator", even if your account is Admin privileged: 32 bit: Link ID=87341 When done installing, set the Windows Defender service to automatic and start it. So for the last month, Windows would fail every time it tried to update.