Download the windows installers from microsoft, make a usb installers and install without using any key.
Now you will have windows for free, but you can't adjust the personalization option, the looks will be default.
And you'll be bothered with an activation watermark at the bottom.
All windows updates and everything will run just the same as the real deal.
If you can't live with that, you can get the retail version for $100 or buy a OEM key (ebay, kinguin, gamivo) from $10.