http://answers.microsoft.com/en-us/protect/forum/protect_other-protect_start/i-cant-uninstall-microsoft-security-essentials/3743598b-7b24-476b-ae61-1922f41836d4?page=2 Open File ExplorerBrowse to C:\Program Files\Microsoft Security Client\Locate the file Setup.exe, right click on it and choose propertiesClick on the Compatibility tabAt the bottom of the tab click 'Change Settings for all users'On the next screen check the box for 'Run this program in compatibility mode for:' and choose Windows 7Click OKClick OKFrom the taskbar search box type cmd.exe and right click and choose 'Run As Administrator'This will open an elevated command prompt as AdministratorIn the command prompt window copy/paste or type the following:C:\Program Files\Microsoft Security Client\setup.exe /x /disableoslimitChoose to UninstallOnce this has finished reboot and Microsoft Security Essentials will be gone and Windows Defender should be running correctly.