You'll need to purchase a new Product Key (license) for each installation
of Windows XP installed on a different computer.
Whenever you have a question regarding the Windows XP
End User License Agreement (EULA), its best to go ahead
and read the EULA itself.
Go to Start > Run and type: WINVER , and hit enter.
Your license to use Windows XP permits you to
install one copy of Windows XP on one computer.
A second installation of Windows XP, installed on
another computer, will require another license.
In order to successfully activate Windows XP for the second
installation, you'll need to purchase a second license (Product Key).
The Windows XP license agreement only permits one installation
of Windows XP on one computer with each Product Key.
Also, open XP's "Help and Support" and type: EULA
and click on "Questions and Answers about the
End-User License Agreement".
To purchase an additional Windows XP license, visit:
Additional Licenses for Windows XP Home Edition
http://www.microsoft.com/windowsxp/home/howtobuy/addlic.asp
Additional licenses for Windows XP Professional
http://www.microsoft.com/windowsxp/pro/howtobuy/addlic.asp