I am going to try to change your plan. You will need to buy a Windows activation key for which version of Windows that you are going to use. Since Win-7 dies in a couple of years, go with Win-10. You cannot upgrade either so you will be doing a fresh installation. A simplified way to do a fresh install without losing data is to replace your HDD, install Win-10, then attach your old HDD as an external drive and save from there what you need.