Re: Cannot install ubuntu-desktop
Go to System > Administration > Software Sources
Toward the bottom of the window there's an entry for your CD. Uncheck it.
Click Close and you'll be asked if you want to reload the package list - you do. Now you can install ubuntu-desktop again from the Internet.
I try to treat the cause, not the symptom. I avoid the terminal in instructions, unless it's easier or necessary. My instructions will work within the Ubuntu system, instead of breaking or subverting it. Those are the three guarantees to the helpee.