Depends if the version you buy is the same version as you have installed (OEM, upgrade). If all you want to do is format and reinstal then it wont matter, if you want to instal over the top so your files remain in place then it will.
The alternative is to go to Microsofts site and look for the Key Changer tool, when it tells you to type in the new COA key and as long as the versions are the same it will make your version genuine, no need to reinstal anything.