Windows Defender in Windows 8.1 Will Not Turn On

I have exactly the same problem as above. I am unable to turn on Windows Defender. When I try method 1 to start it manually I get

Windows could not start the Windows Defender Service on local computer.

Error 577: Windows cannot verify the digital signature for this file.  A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source.

I was unable to start Windows Defender when I bought this laptop new a couple of months ago. It simply would not turn on. Then I updated from Windows 8 to 8.1

The Lavasoft antivrius I had installed (when I found Defender would not work) to protect the laptop would not work any more with the update to 8.1

So I uninstalled and reinstalled Lavasoft but it still would not work so I uninstalled it again.  My laptop is now currently unprotected apart from the free version of Spybot Search and Destroy 2.2 which is the only thing I can find that seems to work with Windows 8.1

There appears to be almost nothing that works in the free antivirus protection now, your Microsoft list says most stuff is incompatible or unknown.

I would very much appreciate some help here to get Windows Defender to work, or please could you point me in the direction of some free antivirus software that will work. Or give me some plan whereby I can protect my computer.

Hi Michael,

It would be great if you can provide more information regarding this issue. Please answer these questions for a better understanding:

1)       Are you able to open Windows Defender program on the computer?

2)       If yes, what exactly happens when you turn it ON from its user interface? Do you get any error message?

I would suggest you to try the following methods and let’s know the results.

Method 1: Start the Windows Defender Service Manually

Please follow these steps:

 a)       Press “Windows Logo” + “R” keys on the keyboard to open the “Run” command box.

b)       Type “Services.msc” in the “Run” command box and press “Enter”.

c)       In the “Services” window, search for the “Windows Defender” service from that list.

d)       Double-click on it to open its “Properties” window.

e)       From its “Properties” window, click on “Start” when the service is stopped and also change its “Startup Type” to “Automatic”.

f)        Click on “Apply” and “Ok”.

g)       Restart the computer and check if that resolves the issue.

If the issue still persists, then try the next method.

Method 2: Run Windows Defender as Administrator

a)       From the Windows 8 Start Screen, type “Defender”. Windows Defender should appear in the list.

b)       Right click on the “Windows Defender” icon and select “Run as Administrator” from that menu.

c)       Check if that launches Windows Defender on the computer and check if you are able to turn it ON.

Please get back to us with the results.

I have the same problem - I tried both of your methods & received errors - unable to send screen shots due to word count

You have to go to the action center in control panel

control panel > action center > windows defender

you will have an option there to turn it back on.

I get a 577 error code