If it's the same cd key that was used the first time it was installed I can't see why it wouldn't work now. What version of Windows are you trying to reinstall?
If you format your hard drive then yes you will have to reinstall the drivers for the graphics card, mobo, soundcard etc so make sure you have the drivers handy before you do any formatting.
I guess another question would be why do you want to format in the first place???