Hi Jewon. I'm Greg, an installation specialist, 10 year Windows MVP, and Volunteer Moderator. If you will work with me I will be here to help until the issue is resolved.
See here how to get Defender working:
https://answers.microsoft.com/ko-kr/protect/for...
Uninstall any 3rd party antivirus in Settings > Apps > Apps & Features, restart PC then run the special uninstaller for the old AV from here.
Restart the PC, then type Security in Start Search, open Windows Defender Settings, there and in Windows Defender Security Center fix anything that's flagged.
Next check for damaged System Files: https://www.lifewire.com/how-to-use-sfc-scannow...
If it cannot repair them see Step 10 here to continue: http://answers.microsoft.com/ko-kr/windows/wiki...
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, and is also the most stable method to advance to the latest version. http://windows.microsoft.com/ko-kr/windows-10/m...
I hope this helps. Feel free to ask back any questions and keep me posted. I will keep working with you until it's resolved.
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