[Freebooter]

If your computer did not come with Windows Installation DVD then your computer could have a hidden factory created partition which will have a Windows image for you to reinstall Windows OS. Research your computer brand to find out how to factory reset your computer you can also read user manual of your computer to find out how to factory reset your computer.
If your computer does not have a factory partition and you want to install Windows OS without using external storage drive then learn how to from my Clean Install Windows Os directly from hard disk drive tutorial.