Xbox -> Problem Resolution -> Dvd Drive:

The most common failure for an Xbox dvd drive is the laser. The laser will slowly wear out after many hours of use. As the light emitted begins to dim, the Xbox will begin to give you "Dirty Disc Errors" while playing a game. The problem will be most noticable when using burned media, but will occur with pressed discs as the problem progresses. The only fixes for this problem are to replace the dvd laser unit, or just replace the entire dvd drive.