Compaq Won''t Start - Trying To Start But Can't And Will Continue

My compaq is having problems starting up. It makes a repetitious sound like the hard drive is trying to start but can't and will continue to do so indefinately or until it boots. So I assumed it was a hdd issue, however trying the hd in another laptop worked fine and an alternate hd in my Compaq has the same problem. It does start up occasionally, although it usually freezes half way through booting up, but can occasionally work upto half an hour.