There is no uninstall windows,.
You could delete all the Windows files and folders manually. For instance, my boot drive has these on it:
Directory of C:\
04/06/2017 09:55 PM <DIR> ESD
02/18/2017 11:52 PM <DIR> NST
03/18/2017 05:03 PM <DIR> PerfLogs
05/18/2017 07:59 PM <DIR> Program Files
05/31/2017 10:28 PM <DIR> Program Files (x86)
04/07/2017 09:25 PM <DIR> Users
08/01/2017 09:16 PM <DIR> Windows
If you deleted those that would get 99.9% of Windows, Might be hard to delete all of them though as some are pretty effectively protected by the OS.
The better, cleaner and faster way is as I described. Copy what you want to keep elsewhere, format the drive then copy the saved data back.