This means that you have an Upgrade version of Office 2000, which requires
that you own a prior version of Office (probably Office 97 or 95) to be able
to install it. You do not need to have one of the older (qualifying
products) installed on your XP machine, just have the installation media
from the older product available. Then, when the Office 2000 installer
returns the message that it cannot find a previously-installed qualifying
product, insert the CD1 or disk1 from your older product into the
appropriate drive and point the installer to it.