A walkthrough article at CodeProject.com that shows how to use a combination of Windows Installer (MSI) and ClickOnce to install and update Outlook add-ins.
Quote:
This article answers the question often asked about Outlook Add-In plug-in: Once an Outlook Add-In solution is deployed on the client computer, how can any future revisions to the solution automatically be made available to the client computer seamlessly?. This article attempts to provide end-to-end steps on how the Update Process for Outlook Add-Ins Solutions needs to be handled once the application is deployed on the End-User machine.
http://www.codeproject.com/useritems/Outlook_Add-in.asp