I like old Office too.
open Word 2003, click the Help and then Detect and Repair. That should do it.
If you only want to do that for a certain extension:
You could go to folder options and change file associacion:
Open any folder, go to Tools>Folder options>File types
And for each extension, change the program
Then you can do like this:
Scroll list down to, let's say...
*.doc
And click Open With
Now browse to the Office 2003 folder and choose Winword.
and you can do that to *ppt, pps for Power points, and so on.....