Can't re-install Leopard(10.5) from external DVD-drive...

I'm trying to wipe my hard drive and re-install OS 10.5.5 from the start up DVD's apple sent me. The internal drive is broken and will not accept discs so I had to buy an external USB DVD drive. When I insert the disc and start the install process the computer restarts and stops doing anything when it gets to the gray Apple screen(there is a spinning dial just under the apple sign). The computer stays there till I turn the computer off.

Why can't I re-install? I've tried holding the option key on start up but all that comes up is the hard drive icon, when I click on it it goes right to the desktop.

Is this caused by the External drive?

Thanks for any help you can give!