license migration from VISTA to XP

hi plz helpme
I got VISTA home basic with all my desktop as the new desktops are not coming with XP.
but I’ve to degrade it to XP.
do I need to purchase a seperate license for each desktop or any installing XP with any CD is ok.
somebody told me that as you haev alraedy paid for the VISTA and microsoft has backword compatibility in license also. so Install XP on all the desktops with any XP bootable CD.
what about the CD key,can I install with the same CD key in all the desktops that I’ve purchased long back for my notebook, can I use the same CD and CD key for all the desktops. or I need the new XP license.
please make me clear.