The XBox controller is actually a hacked HID class item. The main changes are that the interface class ID and subclass ID have been changed to 'X' and 'B' respectively. There are a couple of other minor changes but the changed class ID means you need another driver to support it under a normal computer.

It's basically a reasonably quick and simple way of Microsoft ensuring that you don't use XBox controllers on a PC and cheap PC controllers on a XBox.