Hey mate, this is a common problem with the original Xbox, due to their old age. Its due to a rubber belt in the disk dive stretching or braking. You can either fix the disk driver, by obtaining a new belt, or replacing the disk drive.
This tear down will show you how to open up your Xbox;
[guide|1324]
and these links will tell you in detail how to fix the disc driver, if you wish to;
http://www.ebay.com/gds/Xbox-Repair-How-To-Fix-A-Stuck-DVD-Drive-That-Wont-Open-/10000000018195462/g.html
http://www.gamefaqs.com/xbox/915780-xbox/answers/52373-how-do-i-fix-my-disc-drive-that-wont-open
hope this helps you mate.