Just to point out that, maybe, it's not Windows 7 that's successful, but Intel's Core i-something family of processors you can't buy computers with unless they are bundled with Windows 7.
Someplace else I pointed out Microsoft has only 5 clients they really care about: Dell, Lenovo, Acer, HP and Asus.
Why would you want to? It won't be any faster. If you need XP compatibility for something, use regular virtualization or XP Mode.