It depends, most current laptops and desktops already come with a hidden partition with the windows installer. It is a matter of booting the pc by pressing a special key (F11 or F12 depending on the model) and follow the instructions.

If that partition is not there or the hard disk has been changed you can download the iso from the official microsoft website, it is free to download. Then with this iso you can burn a dvd and use it to format it.