You need either an OEM copy of the OS that came with your computer or you need the recovery media. Your hard drive has likely failed or is about to fail. The best thing you can do is take it out, put a new one in, and then install the OS. If you are lucky, your old drive hasn't failed completely and you can still get data off. The more you use it, the more likely it will fail completely and then getting the data off is a very expensive proposition. Replace the drive and get your hands on some recovery media or an OEM version of Windows.