In Outlook 2010.
File>Options>Add-ins
At the bottom, "Manage Add-ins"
Select [COM Add-ins], then hit Go.
untick Avast add-in.
See if that helps with your immediate problem.
You could try to add that Avast add-in again, to see if the problem remains or if that re-insertion fixes it.
I have had problems with the add-in in the past, but not recently (touch wood).