Why not keep Windows 7 and install XP in a virtual machine? That is what I did using VirtualBox. I even have copies of Windows 2000, DOS 6.2, Windows 3.1, and Windows NT 4.0 SP6!
I think it is a fair assumption since you cannot do an in-place upgrade from winXP to win7 that you would not be able to downgrade from win7 to winXP without uninstalling win7 and then installing winXP.