Sure, there's plenty of guides out there on how to make an adapter, but they all involve splicing a usb cable and cutting open an xbox controller cord.
I needed a controller, and my friend left his xbox at my house to mod for him (a few months ago.. heh).
Since the xbox isn't officially mine, I thought it would be pretty mean to cut open the controller cable to make an adapter.. So I got creative.
leaving the controller as it is, I opened up the xbox and tapped directly into the xbox's controller board. I extended the wires connecting controller port #4 and they all connected easily the computer case's front panel USB ribbon, which was about two feet long.
I fed the ribbon through a rear PCI slot and connected the ribbon directly into my mobo, and windows detected the controller by itself, I installed some third party drivers and everything's snazzy.
And I also have a powerful vibrator....