You downloaded file is definitely damaged.
This could be caused by a bad internet connection, or a problem with your hardware. A friend of mine worked several months with a computer which frequently had problems when writing and reading data to or from his harddisk. Obviously the harddisk was damaged, so we changed it, but the problem still existed. Finally he used a hardware testing program and found that a damaged RAM modul was the problem...
BTT
You should try to download the file using another program, like an FTP client, or just using the browser. You just need to know the address of the file (ask the commish) and type it into the browser, and select "Save As". Once it's downloaded, use TUGZip (->google) to uncompress it and see whether that works.