How to build an Office COM add-in by using Visual C# .NET
http://support.microsoft.com/kb/302901
Microsoft Office XP, Microsoft Office 2003, and Microsoft Office 2007 support a uniform design architecture for building application add-ins to enhance and to control Office applications. These add-ins are called Microsoft Component Object Model (COM) add-ins. This step-by-step article discusses Office COM add-ins and describes how to build an Office COM add-in by using Microsoft Visual C# .NET.