If you bought an upgrade (not full) version of Office 2003
Professional, and if your old Office 2000 was the qualifying
product for upgrading to Office 2003, then your old Office 2000
becomes part of your license to use Office 2003, and you should
not dispose of or otherwise give away your old Office 2000.
If you bought a full (not upgrade) version of Office 2003, and if
you uninstall all Office 2000 applications from your computer,
then you can do what you wish with your old Office 2000. Note
that some editions of Office 2000 include applications (such as
Publisher) that have no equivalent in Office 2003 Professional.
