You have both A)Your Leopard partition set as active (if you have another OS installed on another partition, insert your install disk, open terminal, type fdisk -e /dev/rdiskX, where x is the drive number, then flag Y, where y is the partition # with the other OS) and B)No bootloader on said partition. You should be able to boot off the disk, just don't hit any keys when it says to hit any key to boot off disk.