XP Home doesn't have certain features, like support for 2-way multiprocessing and more advanced networking, but otherwise, they're the same OS. I'm just trying to fathom why you'd want to go to Home from Pro, because there's no advantage of doing so. They both use the same code, unlike Windows 98 and Windows NT/2k.
The only way you're going to be able to do this is going to be by formatting your hard drive and installing XP Home.
Again, I don't see any advantages of going from Pro to Home.