When you open Windows Defender, it may inform you that Windows Defender protection is not enabled in the computer. And, when trying to start the Windows Defender service via Services MMC, the following error may appear:
The Windows Defender Service service on Local Computer started and then stopped. Some services stop automatically if they are not in use by other services or programs.
This can happen if there is a third-party anti-virus software present in the system. It is possible that you installed a third-party anti-virus software earlier and disabled the appropriate services related to that software. To resolve this, uninstall the third-party anti-virus software completely in your system so that Windows Defender can start successfully.
In some cases, the user needs to run the specialized removal tool supplied by the third-party anti-virus software vendor, although the product may have been uninstalled already via Control Panel – Programs and Features. The Experts Community Wiki has the list of anti-malware product removal tools with links to respective anti-virus software vendor sites for your convenience.
Note: After running the appropriate removal tool, restart Windows and enable Windows Defender real-time protection. If you get a different error message, post back in the Comments section below.