Unable to turn off Passive Mode

Ever since installing Avast Business AV Pro Plus, it has been displaying the message “Passive Mode is turned on”, and suggests “another antivirus is installed”. I uninstalled AVG (and rebooted) before installing Avast. There are no AVG subfolders under C:\ProgramData, C:\Program Files or C:\Program Files (x86). I have reinstalled Avast but cannot get around the problem.