I have a Macbook running intel core duo cpu. unfortunately, both my hdd and dvd rom failed. I replaced a new hdd and try to resintall the mac ox (recovery disks, 10.4 tiger, i have the snow leopard disk too) with a external usb dvd drive. I tried both to press the c-key or the option key while turning on. Only the Mac os X install disk icon came up but failed to reinstall. Please help!