If we are talking about the Explorer Extension (i.e. the "Right click on drive icon"), you would have to edit the "Explorer Extension" task from the Enhanced User Interface (avast! Professional only) and change the settings on the "Virus" page - i.e. something you can hardly do by conincidence.
Or, if there was any virus detected previously, you could press "Delete..." and choose "Delete all" from the subsequent dialog. In that case, the next detected files would get detected automatically.
Otherwise, avast! shows the "Virus found!" dialog, asking for the action to take - no automated actions are set.