You can start reading some resources on my web site (below) for add-in
developers.
Once you are somewhat familiar with add-ins, take a look at the
EnvDTE.Project class and its Properties collection (you shouldn´t modify the
file directly).

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Best regards,
Carlos J. Quintero
MZ-Tools: Productivity add-ins for Visual Studio
You can code, design and document much faster:
http://www.mztools.com
>I want to write a plugin for VS net 2005, that will modify the project
> file, set properties in the Property Explorer based on complie type,
[quoted text clipped - 5 lines]
> Thanks
> Wayne