Is the Installation DVD 1 still in the optical drive???

If so the start the computer again with that one and once the installer appears go to the tool bar at the top and choose UTILITIES. Use the Disk Utility to erase the drive and then try to install the system again.

If the DVD 2 is in the optical drive then when you start the computer hold the click of the trackpad, that will force the drive to eject the DVD, then try booting form the DVD 1 and erase the disco and everything else.

If you want to be sure that the DVD 2 is fine then just test in like a normal DVD and see if the system recognizes the DVD, if you can, try to copy some data to the computer.

Examine the surface of the DVD!!