Hi SY. I'm Greg, an installation specialist, 10 years Windows MVP, and Volunteer Moderator here to help you.
Did you have another antivirus at any point? If so make sure it's uninstalled completely at Settings > Apps > Apps & Features, restart PC then run it's special removal tool here:
List of anti-malware product removal tools
Then type Security into Search box, open Windows Security to see if there are any remaining errors to clear. If so report back the verbatim errors or a screenshot following this guide to capture a screenshot and attach it using Attach Image button in reply box:
http://windows.microsoft.com/en-us/windows-10/o...
If problems persist try to get Defender started as shown here:
https://answers.microsoft.com/en-us/protect/for...
If those fail thenDownload, install and run a full scan with the most powerful on-demand free scanner Malwarebytes:
Malwarebytes Download - Free Virus Scan & Virus Protection Tool.
In the Scan Settings first 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.
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://community.box.com/t5/How-to-Guides-for-...
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...
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.
