Need to De-authorize and Erase. HELP!

here is my situation: i picked up an old macbook somebody was throwing away and it still worked, it wasnt not deauthorized by the previous owner and i want to be able to erase and format the hard drive and use it as brand new. The macbook is currently running OS X 10.5.8 and i do not want to upgrade the OS X until the HD is completely cleaned first. I do not own any of the OS X installer discs that may have come with the macbook. My main question is: is it possible to factory reset the macbook so that it is deauthorized and able to used as brand new? or is it possible to nullify the previous authorization by formatting and erasing the HD?