tl;dr: If you recently updated Windows, you should run the Disk Cleanup utility as admin.
Many Windows 10 updates, including the recent "Creator's Update," apparently download and install themselves as a whole new operating system, replacing the existing OS much in the same way my repair installs do. This is a great idea, as it can fix many issues people have in the updating process.
The downside to this is that you now have about 22GB of unused old operating system data on your hard drive or SSD each time Windows has a big update. Windows doesn't automatically clean up that data, and I bet most people don't think to look for it.
Now imagine that you've updated your computer about 4 times as Windows has released large updates. If you have a a 160GB SSD, congratulations! Half the space on your SSD is taken up by OS backups that you didn't make. Yay.
