Unable to load a .NET 2.0 Assembly from VBA in Word/Excel (2000, 2002, and 2003)
Managed code exported via COM Interop may not load correctly inside of Word or Excel VBA if the assemblies were written for the .NET 2.0 Framework. There is a policy that locks back the CLR to versions 1.0/1.1 for Word and Excel, unless the client updates their Office 2003 version or is running Office 2007.
See the following page for the full details:
Unable to load a .NET 2.0 Assembly from VBA in Word/Excel (2000, 2002, and 2003)
