Whenever we purchase Windows, we always prefer to burn its ISO image on a USB or DVD. So that the next time we want to format a machine, or change its hard disk, we will have a bootable media. You will need a 4 GB USB Disk and to be an administrator on the computer you are installing the Windows 7 USB/DVD Download tool on.
Now whenever needed, and you want to format your machine, you will need to boot from this USB device. So what you will need to do is to make sure that the boot order is setup properly in the BIOS by setting the USB boot priority to be higher than the machine Hard disk.
Once this is properly configured, then plug your USB device into the computer and reboot so that your computer boot from the USB device. The Windows Setup wizard will start and you can continue reading How To Install a Fresh Copy of Windows 8 to know how to step-by-step format your computer.