Hi Hafiz. I'm Greg, awarded MVP for eleven years, Volunteer Moderator, and Independent Advisor here to help you until this is resolved.
You said you removed another anti-virus. To make sure it's completely gone run the Uninstaller for it from here: https://www.bitdefender.com/consumer/support/an...
When Defender won't start we next need to check for infection by the most thorough methods, then for damaged System Files, and finally with known fixes for Defender. Please follow all steps in order. I am standing by to help you.
- Download, install and run a full scan with the most powerful on-demand free scanner Malwarebytes:
https://www.malwarebytes.com/mwb-download/.
In the Malwarebytes Settings > Security tab set it to include scanning for Rootkits.
If necessary run it in Safe Mode with Networking, or Safe Mode accessed by one of these methods: https://www.digitalcitizen.life/4-ways-boot-saf...
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 any remainders in Settings > Apps > Apps & Features, and 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
- Then check for damaged System files by running System File Checker from Step 10 in this checklist:
http://answers.microsoft.com/en-us/windows/wiki....
- Now type Security in Search box, open Windows Security to see what the Status of Defender and other protections are now.
- If Defender is still broken, try these fixes next:
https://answers.microsoft.com/en-us/protect/for...
http://www.thewindowsclub.com/unable-to-turn-on...
- Check in Settings > Apps > Apps & Features that no other antivirus' are installed and if so uninstall them now. You may also need to run a special Uninstaller tool for any previously installed Antivirus from here: https://www.bitdefender.com/consumer/support/an...
- If nothing else works then do a Repair Install, by installing Media Creation Tool, open tool and choose to Upgrade Now. This reinstalls Windows while keeping files, programs and most settings in place, is the most stable method to advance to the latest version, and solves most problems. http://windows.microsoft.com/en-us/windows-10/m... .
If you want to keep Malwarebytes as an on-demand scanner then you can turn off its Real Time trial version in it's Settings > Account Details tab.
I hope this helps. Feel free to ask back any questions and let us know how it goes. I will keep working with you until it's resolved.
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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.