Hi, there;
No, I don't think you are doing anything wrong when you try to put your MS Office 2003 Pro suite back.
This is something I have come across quite a few times when I have been rebuilding PC's on upgrades from XP to Vista - mostly, the Office 2003 goes in without a problem - other times, it just wont work and you get incompatability warnings.
... I have not been able to find a way around it except to install the 2007 version of Office - and most people hate 2007 Word once they have been working for years with 2003 Word.
Sorry I cannot be more helpful. Mind you, this is a good excuse to go out and treat yourself to a new quad-core PC with the latest bells and whistles...
If you are good at reading 'specialist instructions', you might be interested in the article at the following URL\ http://uksbsguy.com/blogs/doverton/archive/2007/03/12/loading-office-2003-into-vista-and-fixing-the-problem-with-the-eula-always-coming-up.aspx
... It is a nice and clean site - no nasties or costs...
Regards; Hanfonius