Most likely you will need to buy another copy of Windows. The registry sticker on the side mainly is just to prove you have a legit copy of windows, and maybe for Microsoft support in getting another if the drive were to fail and you needed another copy. I doubt they will give you a copy for a new build. If the registration number has "OEM" in it, it is only good for that computer.
As for what you can do currently, you cant reformat the drive as its the active one (assuming its the boot drive) unless you connect it to another PC and do it there. I would just keep as is, and when you re-install Windows to it, it should give you the option to reformat the drive.