Prior Security Software? What about a trial version of an antivirus program that was pre-installed when the computer was purchased? McAfee, Norton, Trend Micro? If you reformatted/reinstalled the operating system did you also reinstall a trial version
of a security program?
Sometimes remnants of prior security software will suddenly cause issues with your current antimalware program.
A possible reason for this problem is that you have conflicting security software installed fully or partially on the PC. Be sure you have removed all other security software from the PC.
You may need to use a cleanup tool to remove your prior security software: