windows defender not turned on.

[BellaUSA]

Defender is turned off again, don't know why or how it happens. 'Turn on firewall' is checked in both private/public networks Also checked is 'Notify me when an app is allowed thru firewall'. Which should I check, private or public networks ? i never use p.c.'s in public places.

Lenovo 515
Windows 8

[Geek-9pm]

Doing some research on that issue, I found this:

If you have AVG antivirus installed you cannot access any area of windows defender. You failed to mention this before. Windows 8 and 8.1 automatically deactivate windows defender if you install another anti-virus program. If you uninstall the AVG you should be able to access the windows defender application.

Source: http://www.tomshardware.com/answers/id-2225882/turn-windows-defender-back.html
(Near halfway down the page.)
Does that help? 

[Celzium]

You may receive a message: Windows Defender has been turned off, and it isn’t monitoring your computer.

If you have another antivirus software installed, it will turn off Windows Defender. But if you do not and yet , then you might want to carry out the following troubleshooting steps:

1] Open Action Center and see if you are able to enable Windows Defender.

2] Run services.msc to open Services Manager. Ensure that the Windows Defender service is started and set to Automatic.

3] Re-register the following dll files and see if it helps you. Run each command in an elevated command prompt one after the other and hit Enter:

regsvr32 wuaueng.dll
regsvr32 wucltui.dll
regsvr32 softpub.dll
regsvr32 wintrust.dll
regsvr32 initpki.dll
regsvr32 wups.dll
regsvr32 wuweb.dll
regsvr32 atl.dll
regsvr32 mssip32.dll

4] It may also happen that Windows Security Center may prompt you to “Check Settings” for the Malware Protection, and when you click the “Turn on now” button, it may show an error that says “There are no new definitions available to download for Windows Defender”

It may be possible that the issue may have been caused due to an inconsistent WMI repository. Restarting the WMI repository should thus help resolve the issue.

Open Command Prompt as an Administrator. Type the following command and hit Enter:


    winmgmt /verifyrepository

If you get a message “WMI repository is not consistent”, run the following command:

    winmgmt /salvagerepository

This will perform a consistency check on the WMI repository and resolve the issue. Restart your machine.

Try enabling Windows Defender now.

If you get the following message instead:

    winmgmt /salvagerepository is failed

Ignore this message, and again run “winmgmt /salvagerepository”.

Source:http://www.thewindowsclub.com/windows-defender-is-turned-off