I am trying to do a clean install of OSX. I have the original install disk that came with the system. I hold down the "c" button so it boots to the install disk. The screen just sits at the gray apple logo for over 20 minutes. I hear the disk reading every few minutes, but it still sits at that screen. The disk is perfect, no scratches or blemish's. What could be the issue and how can I get this thing to boot?