how do I reinstall windows defender for windows 10

Hi Lucy,

Welcome to Microsoft community.

• How did you uninstall Windows Defender?

Please follow the below steps and check:

 Step 1: Enable Windows defender from control panel 

1.    Press Windows + X, Click on control panel.

2.    On right top corner click on View and then choose large items.

3.    Now from the list click on Windows Defender and try to enable it. 

If not try to enable it from services. 

Step 2: Enable Windows defender from services.msc. 

1.    Press Windows + R, to open run prompt.

2.    Type services.msc and press enter to open services list.

3.    Under services look from Windows defender service and start the service.

If you face any issue while troubleshooting, please provide the outcome, so that we can work on your issue. 

Hope the information helps, if you have any further queries, feel free to post. We are here to assist you.

Rehards,

Guru Kiran

error 577

windows defender cannot be enabled

because of the above error

Defender will not run live, consistently. cp and services of course show the exe as running.

ALL THESE PEOPLE WITH BASICALLY THE SAME PROBLEM TURNING ON WIN 10 DEFENDER AND NOTHING BUT BULL ANSWERS (IF ANY) FROM MICROSOFT. WHAT ELSE TO BE EXPECTED. CAN'T EVEN REMOVE IT FROM MY PC NOW EITHER, NEVERMING GETTING IT TO WORK!!

Tried both methods did not work

second method received an error code # 577

this is the same method described by various people that doesn't work

I came upon a suggestion by somebody that if you installed another virus protection

the windows defender switches off

now what I like to know does Microsoft say that if you have windows defender

you don't need any other virus or malware protection?

Windows Defender is integrated in Windows 10 and cannot be uninstalled. However, when a third-party antivirus app is installed, it will register with the Security Center and Windows Defender will be turned off. This is by design, and is necessary to prevent conflicts that will occur when any two real-time AV apps are running concurrently. When a third-party AV app has been properly installed and you try to start Windows Defender, this is what you should see:

But if Windows Defender didn’t get properly disabled, then you’ll see an “At risk” Windows Defender UI with a message stating that the service is stopped and needs to be restarted. If you then attempt to start the Windows Defender Service in the UI, this will fail because “The group or resource is not in the correct state to perform the requested operation” (Error 0x8007139f).

If you then attempt to start the Windows Defender Service in services.msc, you’ll get Error 577. This is a false alarm, and is merely an artifact of the underlying error state. So there’s really nothing to be concerned about – there aren’t any problems with the digital signature or with malware. You might also have the associated issue where Windows Defender definition updates fail to install with Error 0x80070643. These are all symptoms resulting from a failure to disable Windows Defender in its own registry key when a third-party AV app is installed – as I’ve demonstrated experimentally in this thread:

http://answers.microsoft.com/en-us/protect/forum/protect\_defender-protect\_start/windows-defender-error-code-0x8007139f/93096739-16ce-471b-8716-1776f0c69f89?page=2

So if you want to use Windows Defender as your antivirus provider, then you’ll need to uninstall and run the removal tool for the third-party antivirus product that you’ve installed, or that came preinstalled on the PC. On the other hand; if you want to use the third-party AV app, then you’ll need to completely disable Windows Defender. You might be able to do that just by uninstalling the third-party AV app, running its removal tool, and then reinstalling with the latest version. But that will sometimes fail; and if it does fail (or if you just want a quick fix for this issue), then you should be able to completely disable Windows Defender with the Ten Forms Turn_Off_Windows_Defender.reg file:

http://www.tenforums.com/tutorials/5918-windows-defender-turn-off-windows-10-a.html

The Ten Forums reg file adds a DisableAntiSpywareDWORD to the Policies registry key and sets its value to 1. When I did this manually on the test machine, it sucessfully turned off the Windows Defender UI, and it also fixed the definition update issue – but it didn’t fix the issue with Error 577 in services.msc. So using the Ten Forums reg file to disable Windows Defender with the Group Policy switch is clearly only a workaround for the issue. Nevertheless, I was able to confirm that Error 577 is a false alarm, as indicated above. In order to fully resolve this issue, the DisableAntiSpywareand DisableAntiVirus DWORDs in the Windows Defender registry key (HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Defender) need to be set to the correct value of 1, as I’ve explained in this recent thread:

http://answers.microsoft.com/en-us/protect/forum/protect\_defender-protect\_updating/windows-10-desktop-pc-kb2267602-definition/d5b551bc-bf40-46d8-a05e-924d33882d6d

GreginMich