That's where something like ReactOS fits in. Right now you can legally use a DOS VM running, say, FreeDOS (or an emulator, like DOSBox). There's no freely-available Windows XP equivalent. Sure, sure-- you can run Windows XP illegally w/ a volume license product key (to skirt product activation w/o modifying the product) but that's not a solution that "works" in many cases. It's technically very easy to run a Windows XP VM-- it's very challenging, legally.