For CraigB
I appreciate the additional info! I actually would prefer NOT to have to do an uninstall/re-install, since the particular message I mentioned in my first post has only appeared a couple of other times in the time I've had Avast free installed. I think you are probably correct that it may indeed just be a transitory glitch.
If you absolutely aren't using MS Outlook, then you can uncheck the Outlook options in the Mail-Shield, Expert Settings, Behavior, MS Outlook only section. Then open the MS Outlook, and disable the avast plug-in.
You didn't mention which version of Outlook is on your system (part of MS Office), so here is one to check:
Outlook 2007 > Tools > Trust center > Add-ins >
If you have a different version then you will have to check the help file.