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Hi Emre. I'm Greg, an installation specialist, 10 years awarded Windows MVP, and Volunteer Moderator, here to help you.
No expert in Forums - where we see the most cases, by the thousands - recommends Avast, AVG. Norton or McAfee, but we have solved many problems over the years by having them uninstalled.
I would uninstall any 3rd party antivirus in Settings > Apps > Apps & Features and try running only built-in Defender which gives adequate protection, best Windows performance, least issues, and is from Microsoft who knows how to protect their OS best.
After uninstalling run the cleanup tool for the AV here:
https://www.bitdefender.com/consumer/support/an...
Restart the PC, then type Security in Start Search, open Windows Defender and Firewall Settings, there and in Windows Defender Security Center fix anything that's flagged.
If you want the best on-demand scanner download and install Malwarebytes from https://www.malwarebytes.com/mwb-download/
In it's Settings > Protection > Scan Options enable Scan for Rootkits.
Then on Scan tab choose Threat Scan and Run Scan.
Clean up anything found, restart PC and then run again until it comes up clean.
If you want to keep Malwarebytes as an on-demand scanner then you can disable its Real Time trial version in it's Settings > Accounts tab.
Next check for damaged System Files by running System File Checker from Step 10 in this checklist:
http://answers.microsoft.com/tr-tr/windows/wiki...
Also see these fixes for 100% CPU usage:
https://www.makeuseof.com/tag/fix-high-cpu-usag...
https://www.kapilarya.com/fix-windows-10-high-c...
https://www.intel.com/content/www/us/en/gaming/...
https://answers.microsoft.com/tr-tr/windows/for...
If this is not enough then move to Step 11 to do a Repair Install which saves your files, apps and settings while reinstalling Windows, solves most problems.
I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I resolved your problem, 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.