If you have an OEM copy of Office XP (in other words, it came
preinstalled on your old computer by the computer manufacturer),
then the End User License Agreement does not allow you to
transfer to software to your new computer.
If you have a retail copy of Office XP, and if Office XP is still
installed on your old computer, then you can use one of these
utilities to retrieve the product key from your old computer:
Belarc Advisor
http://www.belarc.com/free_download.html
Magical Jellybean Finder
http://www.magicaljellybean.com/keyfinder.shtml
With most retail editions of Office XP, the End User License
Agreement allows you to install the software on one desktop
computer and one laptop computer (but not on two desktops, nor on
two laptops).