While any number of COM interop assemblies may exist for a particular type library, only one COM interop assembly is designated as the PIA. The PIA contains the official description of the unmanaged types that are defined by the publisher of those unmanaged types. Additionally, a PIA is always digitally signed by that publisher.
Generally, you must avoid the use of any Office XP COM interop assembly that is not provided as part of the Office XP PIAs. You must also avoid the use of any Office XP COM interop assembly that is generated by Microsoft Visual Studio .NET at design time. Any Office XP interop assembly that is not included in the Office XP PIAs download is considered unofficial.
To download the official PIAs available to Microsoft Office XP applications and associated technologies, visit the following Microsoft Download Center Web site:
Article ID: 328912 - Last Review: Mar 23, 2009 - Revision: 1