Should be on the back of your CD. If not there or on any papers you have with it, you may be able to find the existing key if you have it installed already. Install Magic Jellybean Key Finder, run it, will get you the setup key for it.
If you don't have a key for it, need to contact Microsoft. Although not sure if they will offer any help for Office 2000. At least not for free.
As an alternate, you can try openoffice. www.openoffice.org