I replaced the hard drive on a mac mini and needed to reinstall OS X. Unfortunately, my copy of the Leopard DVD would not boot the mini, or my MacBook Pro -- the computers both just spun it up and down multiple times before ejecting it and displaying the circle-with-a-slash symbol. On closer inspection, the DVD was scratched, but would still appear fine within finder. This is my work-around to install using an iPod.
Step 1: Make an Image of the DVD
Make an image (.dmg file) of the DVD. I did this through Disk Utility.
Step 2: Copy the DVD to an Ipod
Copy the DVD's image to an Ipod. Within Disk Utility, I erased an 8 GB Ipod, then restored it using the Leopard image.
Step 3: Boot From the IPod
Connect the iPod to the mac mini via USB, and boot the mini while holding down the option key. The Leopard install DVD iPod will appear; click on it to boot.
Step 4: Install Leopard
Install Leopard as normal.