Is your Xbox modded Drew?
How can you download a game from the internet? I'm so out of the loop as to exploits on the 360.
There are 2 types of mods for the 360:
1. iXtreme - This is the process of extracting your DVD drive key, and reflashing the DVD rom with custom firmware. This is the most basic of mods and allows the user to run DVD-R DL discs. You're not modding the OS, you're going after a single component, the DVD rom drive controller.
2. JTag - This is the process of extracting your CPU key and reflashing an entirely new OS to your 360. The OS looks identical to the OEM MSFT one, but has all security torn down. You have the ability to run "unsigned", DRM free code ... so if the data is in the proper format, you can play it. (I have entire Rockband track libraries, full XBL arcade lineups, etc)
JTag is an extremely rare modification. Not every console can have this done because the exploit was patched around 2 years ago. My Jtagged console was purchased new back in early 2010 from old, old store inventory. The modder executed the hack (soldering, custom USB port building, etc) and sold the machine.
Reach leaked online and it's for JTag consoles only. It looks like what was stolen was the final files for disc pressing. Looking at the file structure / directories of Reach, this came off of a Bungie employee's work station. What's out there are the finalized, but raw data files.