Could it perhaps be the DVD Drive - how can test to check if it is?
Generally, if any disc works, it's safe to assume the drive is fine too. Have you tried using other discs in the drive in question? If you have, and other discs work, please list all components of your system build. Provide as much part information as you can, such as:
make and model, interface (IDE or SATA), etc. Also please tell us why you're trying to load the os disc (did the system not come with an OS installed?). There could be a compatibility issue here.