messaging interface has returned an unknown error when accessing the contacts in Outlook 2003

[John]

I'm getting the following error when I attempt to create a new email message
in Outlook 2003. This happens when I click on the to to open up the address
book.

The messaging interface has returned an unknown error

I'm running the latest updates for Office 2003 and this only happens to one
user accessing his MS Exchange contacts. If I have another user create a
message and selecting the to for his outlook contacts using Outlook 2003 on
the same computer there is no probelms.

If I bring up outlook using the outlook /safe I can access the users
contacts with no problems. Seems there is some third party application
causing my problems. Any idea where to look?

Thanks