|doesn't sound like avast installed properly. The Bootscan takes quite awhile to run, depending on HD size and content.|
Did you by chance have norton or mcAfee installed? If so, you may want to go to their websites and use their uninstaller.
You can also try uninstalling avast and installing it again and once again do a bootscan on reboot, that should find whatever Malwarebytes may have missed.
Here is the uninstaller for Avast:
Some HELP in posting on Computing.net plus free progs and instructions Cheers