You can find how Jtag your Xbox on Youtube, but this requires you to have some soldering skills. There are things that slot between the xbox and the HD, you just need to google xbox 360 mods.
Just remember, if MS Live find out you are on a DL'd game they will cancel you Live account. Apparently on some modded xbox's you cant do any updates on xbox live without giving away you're on a DL'd game.