Deleted the Windows.old folder and got another 200GB. Not sure what that folder was even doing there, it was supposed to reset the PC, guess I should've selected the "in case you're recycling your PC" option...

It keeps the old files as a backup in case something goes wrong and you want to roll-back the system. If that's not an issue for you, you can just nuke it.   

@MEC-777 might as well...