Unable to enable Windows Defender application in Windows Vista

1. Click Start, type CMD in search.
2. From the results, right-click Command Prompt and Run as Administrator. Type the following command: (If you have logged in as Administrator, you can directly execute CMD)

winmgmt /verifyrepository

3. When the system returns "WMI repository is not consistent" message, run the following command:

winmgmt /salvagerepository

This will perform a consistency check on the WMI repository and resolve the issue. Restart the system, once done and try enabling Windows Defender program.