1) Office XP (or any other software CD for that matter) do not have some internal database storing all the legitimate keys. Instead, when you enter a CD Key, they run complex algorithms to check if the key you entered generates a code that WOULD HAVE been generated by a valid key.
In other words, the CD just contains the software itself. You need to get a valid key from the publisher / vendor of the software.
2) There are utilities that can extract CD Keys that were used to install Windows / Office on a machine. If you installed Office on another computer, you can find the key using a utility such as CD Key Reader.
http://skaro.net/
3) Remember though that you can not use Office / Windows XP on more than one machine. (Technically you can install, but you won't be able to Activate it and it will eventually get disabled.)
4) Also, OEM versions of software are bundled with PCs and they may not even install on another brand / hardware machine. Retails versions are portable (but again subject to being used on one machine at a time).
If this information contributes towards resolving (or increasing your understanding of) your problem / issue , please post back a reply for others' benefit as to what worked. While you are at it, rate the answer(s) as well !