"Ask Microsoft if they will place the XP code base into the public domain and let others support it."
XP is a real mess, disentangling that code would be a total nightmare!
You'd do better supporting ReactOS -
ReactOS Project
Win7 is much cleaner and more stable
Bite the bullet!
Mac
PS: If you install Win98SE in a VM and strip it down you can make it pretty small and surprisingly stable - but of course it is totally insecure, don't let it near the 'Net!