Windows Defender 0x80070003: The system cannot find the path specified.

• The Event Viewer may contain the following entry:

  Windows could not start the Windows Defender Antivirus Service service on the local computer. Error 2: the system cannot find the file specified.

• The Defender service itself may show as stopped in the services console. Trying to start it results in the following error: Error 126: The specified module could not be found.
  

• Also, the following error codes may be displayed in Server Manager or visible in Event Viewer System or Application logs:

  0x8007007e: Antimalware protection has stopped functioning for an unknown reason.

  0x80070003: The system cannot find the path specified.

• Standard repair commands such as the ones listed below fail to resolve the condition:

  SFC /ScanNow

  DISM.exe /Online /Cleanup-image /Restorehealth

A possible cause for this behavior is an unintended deletion of the following folder (or contents of it) on the affected system:
C:\ProgramData\Microsoft\Windows Defender\Platform\<Platform Version>

This may be the result of aggressive settings of an additional third-party anti-virus software installed on the system.

If the C:\ProgramData\Microsoft\Windows Defender\Platform\<Platform Version> directory is missing or empty, you can copy it (and its contents) from a system with a similar operating system and Windows Defender platform version onto the affected system.

After this and a reboot, the Windows Defender service starts.

If a second, non-Microsoft anti-virus software is present, follow the guidance and best practices documented here.