<p>I need to open a Microsoft Word 2003 file and change its file properties.  Such as changing the Subject in the Summary Tab. <br/>
<img src="https://i.stack.imgur.com/FEpJY.gif" alt="alt text"></p>

I can think of 2 ways to do this:
Use the Microsoft Office APIs. You
will have to reference them in your
project, and you will need the
Primary Interop Assemblies
.
Convert the file to the Word 2003
XML format and change that value in
the XML document. Here is the MSDN
documentation on the document
properties:
http://msdn.microsoft.com/en-us/library/aa223625(office.11).aspx
I would go with the second option if you can, because that way you don't have to depend on Word being installed on the system.