I'll just leave this here, then leave the thread...
http://www.microsoft.com/OEM/en/licensing/sblicensing/pages/what_to_do_downgrade_rights.aspx
Q. Where does the system builder or end user get the Windows Vista Professional, Windows Vista Ultimate, Windows Vista Business, Windows XP Professional, Windows XP Tablet PC Edition, or Windows XP x64 Edition media disk to perform the downgrade?
A. The
downgrade media can be media associated with any prior legally licensed version from the OEM system builder or retail channels. Additionally, an end user, who is licensed separately through Microsoft Volume Licensing programs, may provide their Volume Licensing media and key to their system builder to use to facilitate the downgrade on only their systems.