World Icon on startup



I have a copy of osx 10.4 tiger for my powerbook g4, I boot the machine with the install disk and a blinking world icon appears followed by a small folder and question mark icons that alternate between eachother which then stops alternating leaving the small folder permanently showing.???

Any suggestions, apple gurus think it's the optical drive, I checked inside and the drive is properlly connected and ejects and excepts disks fine.