I've had this problem a lot lately, don't know why it just started.
However, it seems to only happen when creating a new outlook profile, or installing outlook for the first time.
Just let it disable the add-on.
Close Outlook.
Re-open Outlook. There should be no errors now, and the Avast Plugin will be disabled.
Go to disabled items (in Outlook 2003, it's Help > About > Disabled Items) (In Outlook 2007, Help > Disabled Items) and re-enable the avast plugin.
Close Outlook. Re-open, and the problem should be gone, and the plugin should be running.