Thank you,
there is one lead that might indicate what is going on. I cannot reproduce the problem here, so it is just my guess, anyway I made a change to the code which might help. Please test it in the next beta refresh (due out after Easter Monday).
Report back if that resolved the problem.
If the toolbar does not appear, ensure that you check View > Toolbars > avast! Antispam checked and try again.
If you still have no luck then close outlook and go to Avast > Settings > Maintenance and enable debug logging. The new build of asOutExt.dll (not the current 5.0.462) supports verbose debug info. Start Outlook, wait for it to fully load, close it, wait for it to shutdown and then post the asOutExt.log (which should now have extra debug information about the plugin's lifecycle)