1.) Place the 10.5 Install disk in your drive.
2.) Restart your Mac (don't use on the installer on the disk)
3.) After the chime, hold down COMMAND+OPTION+O+F until you see a light gray screen with black text
4.) let go of the keys and type in the following commands EXACTLY as shown, and hit the RETURN key after each line (you will see an "ok" confirming what you typed was correct):