FOR PEOPLE WHO CAN'T ACCESS TASK MANAGER(Crash) or SAFE MODE, I HAVE FOUND ANOTHER METHOD..
After login, Press Ctrl+Alt+Del then, click on the Power button. It'll show some error, click OK.
Then, there will be an Error Report. Click on More Details, Scroll down & click on the last Hyperlink (Blue, underlined test). This will open text report in Wordpad.
Click File -> Open. Select All Documents from the dropdown menu at the right-bottom of the window.
Browse to "C:\Program Files\AVAST Software\Avast" & look for AvastUI, Right-click on it & Run as Administrator..
Now you can add the explorer.exe & SystemSettings.exe to Behavioral Shield exceptions as mentioned everywhere.. Restart...
Enjoy!!