Does the computer think that your DVD Rom is a CD Rom in Hardware Manager or in My Computer?
Seen lower end combo DVD-CD Drives do this before if generic brand, and you need to instal driver for drive to be recognized as a DVD Rom drive and not just a generic CD Rom Drive.
Would help if you could open your system to give us the model # of the DVD Rom, as for there are tens of thousands of different DVD Drives out there.
Dave