This sounds familiar. I don’t think this is problem with avast. I think it’s just a setting that shows a popup whenever a file is scanned by avast’s on-access scanner.

Right-click on the avast icon in the system tray and go to On-Access Protection Control > Standard Shield > Customize > Advanced & uncheck the setting “Show detailed info on performed action.” Then, OK your way out of the settings & your problem should be fixed.