As above.
Would've said try it in a different computer next, but I had to go, it was late.
Suggested by WillyW (in a PM to me) - are you putting the disk in the right way up?
Shiny side down, that is.
Might sound silly, and no offense intended if it comes across like that, but it's best to try and eliminate every possible cause of the problem, no matter how trivial.