You will need to reinstall your OS to the HDD. Just moving a previously installed OS on a HDD to another computer will not cut it. The license will probably be a Dell OEM which means you will need to buy a new license for your new Rig. Save a bit of money and get Win 7 Pro. If you own Win 7 ultimate retail then you can transfer the license but you will still need to reinstall the OS. If the license came with the Dell you bought then the license belongs to that Dell and therefore cannot be moved to another PC strictly legally speaking.