I tried to start my computer using the installation disk (holding down the c key) to try to repair my disk using the installation disk - but instead of booting up the computer - it said OS x can not be installed on this computer - and gave me the option to back up using time machine or restart - so I never could access the disk utility - am I missing something in trying to start the computer with the installation disk? I held the c key down the whole time, and I also tried just holding it down until the gray apple appeared - both with the same results.