This month, Microsoft released its first major update to Windows 10. If you've already done this upgrade, be sure to run Disk Cleanup. It can save you upwards of 20GB of hard drive space.
Those of us at Lifehacker who have already installed the Windows update noticed that during a Disk Cleanup, there's an easy 20GB of space that can be removed. To find it, run the Disk Cleanup utility, and after the initial scan, click "Clean up system files." After a second scan, there should be an option reading "Previous Windows installation(s)." When we ran the scan, this item was not selected by default, but it contained a massive amount of data. Go ahead and clear that out to get your space back.
Of course, those previous installation files may be necessary if you need to rollback to a previous version of Windows, so if you're not sure that everything will work on the November update, feel free to keep it around until you're sure. Once you know everything's working, though, clear it out.