What a fuss! For Heavens sake!
Virtualize your old XP machine and run it in a VM on a modern OS (Win or *nix)
Just don't give the VM access to the Net - or let it and just revert to a known clean snapshot everytime you run it.
You could use a non-routable protocol like NetBIOS/NetBEUI or DLC to communicate.
Paragon's Go Virtual works nicely -
Virtualize your computer (make a virtual copy of it) with Paragon Go Virtual | Reviews, news, tips, and tricks | dotTech
So does VMWare -
Virtualize your old Win XP machine on Windows 7 | DigiBlog
Anyone would think the skies were falling!
Mac
Edited to add:
I don't of course mean that folks should still be running XP as their main OS.
They shouldn't.
BUT if you're keeping it around to run some very specific app then there might be some rationale for keeping an old copy around and there are ways of doing this in reasonable safety.