Thats a strange issue, if you can read them on the disk utility. Try booting from the mac os installation disk, by booting and holding "C" if it will boot off of the disk, then there is a problem with the operating system. if it won't boot off of the disk, then there may be a problem with the optical disc drive, open disk utility and select "Verify Disk" under the optical drive, if there is an error, then the drive is almost certianly bad.