Power Mac Boots to old hard drive but wont boot to install cd, Kernal

Just bought a power mac g5. The machien will start up to the install of 10.5 that the orginal owner has but will not boot into the install cd of 10.4, or 10.5. I tried reseating the ram and removing some with no help. I also tried booting from firewire and usb but the machine will not recogonize either of these when i hold down "option".....when i go to try and boot from the cd it will pop up with the string of Kernal information and tell me to reboot. but then if i take the cd out it will boot into the orginal OSX just fine...