If I replace my hard drive do I have to re install windows 7 from using my product key sticker which is found under my laptop ?

Yes you have to reinstall Windows 7, because the operating system resides on the Hard Drive. You can certainly try using the product key under the laptop. I also believe that if you have the recovery media and want to reinstall on the SAME computer with a different hard drive it should work. It is possible that you will not have to register again.