Are you sure? I mean, the OE progress bar has always been unusable with the avast mail scanner turned on (as far as I can remember), that is, it has always been displaying the progress of the local communication "avast --> OE" (which is naturally very fast), as opposed to "mail server --> avast" (which is what you'd like to see).
Haven't you been using Outlook (not Express) instead? In Outlook, scanning is performed differently and the progress bar works.
Thanks
Vlk