Start the Process - Alternative Method
The alternative way to start the install process is to boot directly from the DVD, without first mounting the installation DVD on your desktop. Use this method when you’re having problems and you’re unable to boot to your desktop.
Start your Mac while holding down the option key.
Your Mac will display the Startup Manager, and a list of icons that represent all of the bootable devices available to your Mac.
Insert the Leopard Install DVD into a slot-loading DVD drive, or press the eject key and insert the Leopard Install DVD into a tray-loading drive.
After a few moments, the Install DVD should show as one of the bootable icons. If it doesn’t, click the reload icon (a circular arrow) that’s available on some Mac models, or restart your Mac.
Once the Leopard Install DVD icon displays, click it to restart your Mac and boot from the installation DVD.