Here's where you can go.
http://www.microsoft.com/products/info/product.aspx?view=22&pcid=a9d2c448-eb05-4a2b-a062-9c711c533e0c#
Windows XP additional license pricing
Windows XP Professional?full version $269.00
Windows XP Professional?version upgrade $184.00
Windows XP Home Edition?full version $184.00
Windows XP Home Edition?version upgrade $84.00
From a licensing standpoint, these are the rules as posted in the EULA from my copy of XP Pro (eula.txt):
9. UPGRADES. To use Software identified as an upgrade, you
must first be licensed for the software identified by
Microsoft as eligible for the upgrade. After upgrading,
you may no longer use the software that formed the basis
for your upgrade eligibility.
13. SOFTWARE TRANSFER. Internal. You may move the Software
to a different Workstation Computer. After the transfer,
you must completely remove the Software from the former
Workstation Computer. Transfer to Third Party. The
initial user of the Software may make a one-time
permanent transfer of this EULA and Software to another
end user, provided the initial user retains no copies of
the Software. This transfer must include all of the
Software (including all component parts, the media and
printed materials, any upgrades, this EULA, and, if
applicable, the Certificate of Authenticity). The
transfer may not be an indirect transfer, such as a
consignment. Prior to the transfer, the end user
receiving the Software must agree to all the EULA terms
So I would say that yes you can move it as long as you remove it from the other computer. If you want the official version Microsoft has a presales/licensing line: 1-800-426-9400
--Delz