Your post really makes me ask the question 'Why?'
What he said.
even hough I've already paid for it.
Is it per chance an OEM copy of XP that is tied to the previous chassis ? In which case, yes you have paid for it, but no, you do not have rights to use it on another machine.
I would suggest if you really want XP, then just reformat the hard drive and install XP on it, and then cross the Windows 7 bridge as and when you feel like it.