Windows 10: How do I reset/repair Windows Defender?

About a week ago Windows Defender detected several malicious files inside an ISO file on my system. I didn't even know Windows Defender would scan inside ISO files. Either way, I manually deleted the ISO file from my system. Now my Windows Defender is stuck in a bad state. It still says these malicious files exist, but any attempt to quarantine/remove/allow the file results in "Remediation Incomplete." I have rescanned my system, updated Windows Defender, performed a full scan - nothing I do will get me out of this state. Windows Defender still thinks this malicious file is on my system

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What the hell, windows defender?

Installing major updates like the April update will reset pretty much all Windows features including Windows Defender.

 

how to reinstall windows defender?

The system is infected by miners and other malware, which have deleted your Defender executables completely. A repair installation is suggested.

Before that, please do the following:

• Download FixList.txt
  

• Save Fixlist.txt in the same folder where EnglishFRST64.exe is.
  
• Close all program windows.
  

• Launch the Farbar Scanner tool and click "Fix".
  
• Upload the output log file (FixLog.txt) to your OneDrive.
  

Once done, repair Windows 10. See How to perform an In-place Upgrade with Windows 10 Step-by-Step Guide - Microsoft Community:

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Windows 10 Advanced Options Menu to Repair

I would try the following steps in order:

1. Remove the laptop main battery for a few minutes - may reset something
2. Load BIOS defaults
3. Install Macrium Reflect Free on another PC, create a Reflect USB repair disk then use that disk to repair your laptop
4. Recover from backup if you have one
5. Perform a Windows 10 repair install - see instructions on this forum