Microsoft has released two developer utilities for Office XP

Office XP Developer Utility: Manged COM Add-In Security Shim

Deploying and installing a managed COM add-in securely in Microsoft Office XP currently requires the assembly be hosted in an unmanaged COM add-in proxy component called a shim. This COM add-in shim solution template, properly configured, allows you to successfully deploy your managed COM add-in under the highest Office XP security setting. For more information on configuring the shim, see the MSDN technical article Using the COM Add-in Shim Solution to Deploy Managed COM Add-ins in Office XP.

Download

Office XP Developer Utility: Managed Smart Tag Security Shim