VSTO Outlook AddIn disabled in Vista

Question

We have an issue with an Outlook 2007 AddIn (created on a Windows 7 development machine with Visual Studio 2008/VSTO).

It installs with the MSI installer on these machines:
- the development machine
- a Windows XP machine

Install initially succeeded on a Vista machine, but after installing an updated version of the AddIn, it gets disabled when starting Outlook. We suspect it might be related to how Vista handles security.

Have anyone encountered issues specific to installing VSTO AddIn on a Vista machine?
(Please note that the AddIn in our case gets installed alright, but is 'just' disabled when starting Outlook.)

Answer 1

The add in might be throwing an exception. An unhandled exception causes the add in to be disabled.

Answer 2

We hit this issue on some random machines and after much head scratching, it turned out that Office.Interop was not installed on the client. We had assumed it would be installed with Office 2007 - which it appears to be in many cases.