Hello,
i had the same problem with a colleague of mine.
The trick is to uncheck the Outlook Addin.
What Outlook Version do you have ?
In 2007 it is (i hope i translate it right - i have the german version):
Tools/Trust Center/AddIns
Then select Exchange Addins at the bottom of the settings window.
There should be something like "Symantec Client Security 3.1"
If you uncheck this. The error should no longer occur.
If you can't find it this page will help (for Outlook 2003):
http://office.microsoft.com/en-us/outlook/HP052427821033.aspx
Greets
Stephan