The Performa 637CD is one of them older machines that uses an archaic key combination to boot from a bootable CD. In fact, if it's a pre-PPC Mac, it takes an finger gnarling key least that much I can remember. If ya thought the "three finger salute" to force-reboot a Mac was bad...the key combo to boot a 68K Mac from a CD is even worse...IF I could just remember it.