PPT/Office 97 (and 2000) uses the old Org Chart 2.0, whereas PPT/Office 2003
(and 2002/XP) uses a new diagramming applet. If the older version of
PPT/Office has never been installed on the system in question, then Org
Chart 2.0 probably also hasn't been installed.
IIRC, PPT 2002 (Office XP) would convert old org charts to the new type of
diagram when you'd double-click them. Problem is, the conversion was often
pretty bad. I believe that's why PPT/Office 2003 doesn't try to do the
conversion, and so you have to install the old Org Chart 2.0 application in
order to edit Org Charts.