See
http://support.microsoft.com/?kbid=828450 "Why service packs,
updates, and security patches may require the Office 2000 CD-ROM".
Even if you use the full-file version of a service pack for
Office 2000, you may still be prompted to insert the Office 2000
CD. If you cannot provide the CD when prompted, then the service
pack installation will fail, and some of your Office applications
may be left in a non-functional condition.
You should keep your Office CD in a safe place. You will need it
to install updates, or to add or remove features, or to repair or
uninstall or reinstall Office.