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)