Visual Studio Tools for Office is pretty neat, but keep in mind that you
don't need it to automate Office 2003. VSTO lets you automate particular
documents -- it lets you write VB.Net/C# code that "runs behind" a document
(or document template.) E.g., a sales report document in Excel that
automatically downloads data from a server. Microsoft calls this a
"document-centric" perspective.
You can still use "classic" automation without VSTO -- either by developing
a COM add-in, or by automating an Office application using the Office object
model. I'm developing a COM add-in for Word with VB.Net, and there doesn't
seem to be any real difference between earlier versions of Word and Word
2003 in this regard.
Now if only I could customize Office 2003 to get rid of those hideous blue
menus and toolbars.... <g>