|If you have XP installed already on a computer - and that is where you wish to re-install XP at least - then that information is usually on a MS style sticker on the back of the desktop unit, or on the base of a laptop (occasionally inside the battery compartment).|
Pre-installed XP will be an OEM version (usually), not Retail. That OEM key will (only) work with most OEM versions of the XP setup disk; Retail only with retail versions.
Are you trying to re-install XP and Office, starting from scratch. Is XP already installed and is Office already installed - both on the same computer to which you wish to re-install them?