Try reformatting the Xbox. if you have save games you need then find a friend with a memory card and put them on there, then you can revert to factory settings.
Alternatively, there is a way to clear out your cache - go to the pane with memory on it. Select your hard drive, and press the Y button. now, it'll give you an option to delete or cancel or something. INSTEAD press X, X, left bumper, right bumper, X, X and it'll bring up a hidden option.
This option will clean up all cache files as well as delete any game updates.
If it all goes tits up, then please don't hold me responsible :)