Hi, if my hard drive dies and since the Vista OS is saved on the harddrive, If i were to buy a harddrive from an electronic store, how would I go about getting the Vista OS and Drivers to install to install? How much does it cost?

If i were to buy a hard drive (250gig 5600) from Dell, would I have to pay for the Vista OS again or does Dell just give it back to me? How much does this cost?