There have been several reports of the Windows 10 upgrade from Windows 7 with Microsoft Security Essentials installed has left MSE on the machine and disabled Defender. The uninstall procedures for MSE do not appear to work.
See this Wiki article: Uninstalling Microsoft Security EssentialsThis article contains information that may help if MSE was not removed during the upgrade to Windows 10.
Windows 10 should have recognized MSE as incompatible at installation and removed it automatically. Something apparently prevented that from happening in the cases we've seen here in the forum. I presume that a manual uninstall of MSE may also be problematic in some cases -- either before the Windows 10 upgrade or after a rollback. Note that the uninstall MSE Wiki article (Uninstalling Microsoft Security Essentials ) has been viewed many, many times since publication as it, like other antivirus software, can be balky about an uninstall via normal means.
Based on feedback here in the forum there are several options open to you if you are faced with MSE on Windows 10 and a broken Windows Defender after an upgrade:
a. do a reset of Windows 10, which will remove all installed programs and data and will leave you with what will in essence be a clean Windows 10 installation with no programs installed.
b. roll back to Windows 7, remove MSE, perform the upgrade to Windows 10 a second time
c. manually rip out MSE by editing the registry, stopping services while running in Safe Mode, and possibly removing leftover program files manually.
d. Forum user ElifeGenc provided this link - http://scottiestech.info/2016/04/30/windows-10-cant-uninstall-microsoft-security-essentials-no-problem/ - I've not tried this procedure, but there have been reports of it working.
Special thanks to: