There are features that are hidden in different versions of Windows.
For instance, did you know it's possible to get XP Pro to build a software RAID, something that generally only server based OSs are supposed to be able to do? Yes, it is, and natively too. But you have to enable that feature with registry hacks.
What this means to me is that really, Windows XP Home, Pro, etc. are all to a great extent the same, but with the different features enabled/disabled depending on the version.