You're off topic for this forum, which is dedicated to Windows Live OneCare. You may want to try the Microsoft Public newsgroups here: http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx
However, if your XP license was an OEM license that came with the PC, then you can't transfer it. If it was a full copy of XP, simply activate after installing the OS to the new hardware.
-steve