Hi Minukjaya. I'm Greg, 10 years awarded Windows MVP, specializing in Installation, Performance, Troubleshooting and Activation, here to help you.
- Try these fixes for known error of You'll need a new app to open Windows Defender link" with fixes the same for WIndows 10 and 11:
https://www.kapilarya.com/fix-youll-need-a-new-...
https://michaelspice.net/uncategorized/youll-ne...
https://thegeekpage.com/fix-youll-need-a-new-ap...
https://social.technet.microsoft.com/Forums/for...
https://superuser.com/questions/1671179/how-to-...
https://www.minitool.com/news/you-need-new-app-...
https://www.youtube.com/watch?v=UsgKUhf5Rbg
- If none of those fix it then when Windows Defender is broken the first thing we have to check is that there is no infection using the most thorough method, then check for broken System files, then if necessary the known fixes and finally if nothing else helps do a simple Repair Install:
To check most thoroughly for infection and any resulting System damage, Download, install and run a full scan with the most powerful on-demand free scanner Malwarebytes:
https://www.malwarebytes.com/mwb-download/ Make sure to only choose the Free version.
In the Malwarebytes Settings (gear icon) > Security tab set it to include scanning for Rootkits.
Clean up anything found, restart PC and then run again until it comes up clean.
Then download, install and run a full scan with AdwCleaner:
http://www.bleepingcomputer.com/download/adwcle...
Remove whatever it finds.
Check for anything found but is still left over in Settings > Apps > Apps & Features, and C:\Program Files and C:\Program Files(86) to uninstall or delete them. I can guide you how to do this if there are problems.
Also in each of your browser's Extensions, Home Page settings, Search service or Add-On's as shown here: https://www.computerhope.com/issues/ch001411.htm
to disable anything you didn't add yourself and are sure you need. Ask back if in doubt.
- Next check for damaged System files by running System File Checker/DISM in WIndows 11:
https://www.elevenforum.com/t/use-system-file-c...
- Now type Security in Search, open Security Center and check if all functions are working. If Defender is still not working try these fixes next:
https://thegeekpage.com/windows-defender-wont-s...
https://www.kapilarya.com/fix-windows-defender-...
https://www.digitbin.com/fix-windows-defender-n...
https://www.technipages.com/fix-windows-11-cann...
https://windowsreport.com/windows-defender-not-... (Avoid ad links which intrude into editorial copy, especially avoid Restoro, PC Repair Tool and Reimage ads)
https://www.youtube.com/watch?v=aR0W1ltf3ic
- If none of those work then you can do a Windows 11 Repair Upgrade which reinstalls Windows while saving your files and apps in place, brings it up to the latest version by the most stable method, takes about an hour and resolves most problems: https://www.elevenforum.com/t/repair-install-wi...
If you want to keep Malwarebytes as an on-demand scanner then you can turn off its Real Time trial version using the slider buttons on it's front panel. I recommend it as the #1 tool for your toolbox. For best WIndows performance, use built-in Defender which gives adequate real-time protection.
Feel free to ask back any questions. Please report back results for all steps given so I know what else to suggest if necessary.
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Standard Disclaimer: There are links to non-Microsoft websites. The pages appear to be providing accurate, safe information. Watch out for ads on the sites that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the sites before you decide to download and install it.