i just bought a macbook from someone and i wanted to clear the information, when i put the OS disc in to reformat it was giving me "Mac OS X cannot be installed on this computer" so someone told me to erase my HD to reinstall it, i erased my HD and tried again and still got the same thing so now my macbook wont boot up because theres nothing on the HD...why is the disc saying that and how can i fix it?