FATE has made a new post: re: The Xbox Operating System.
Q: Is there any way to get ahold of an OS Disk or download the OS from a third party?

A: Yes, you can now obtain an operating system software update by download a file from Xbox.com and burn a DVD or CD.
Follow this link for detailed instructions: http://www.xbox.com/en-US/support/systemuse/xbox360/console/softwareupdates.htm