Does the drive show up in the BIOS?
Disconnect the any possible boot device (hard drive, floppy drive, any additional optical drives...etc), except for the optical drive. Then verify that the CD/DVD drive is still the first boot device. Disable all other boot devices in the BIOS. Then try booting.
That will force the PC to boot off the optical drive or not at all. That will narrow down the issue to either the drive or the disc (which you've already said the disc works).