Hello Rob Knight,
Just make sure the new computer comes with an OEM license for Windows 7 Professional or Utimate, or Windows Vista Business or Ultimate. OEM licenses are permitted downgrade rights.
So strictly speaking you won't be "transfering" the old XP Tablet license to the new computer, because that is not permitted. Instead, you will be installing XP Tablet as a downgrade from the license for W7 or WVista that comes with the new computer.
See this page:
http://www.microsoft.com/oem/en/licensing/sblicensing/pages/downgrade_rights.aspx
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