any xbox (except the new slim version) can have a hard drive installed. On the top of the xbox (if it's standing up) the surface of the top of the xbox should be filled with holes like a grid (and is grey) there should be a grey button that isnt filled with little holes, press that in and the top of the xbox comes off and that is where the hard drive is placed, i have an xbox arcade and i hav a 60GB hard drive on it
Take a look at this pic:
http://www.game-consoledoctor.co.uk/media/editor/image/xbox360_4.jpg