Anyone who finds their hard drive packed to the gills after installing Windows 10 should check and see if their old operating system is still tucked away on the drive and then think about whether they really need it there.
The old system is automatically stored on the drive because Windows users are supposed to be able to access it for one month after installing a new operating system in case problems crop up. But, if you're sure you don't need the old system, there are ways to turn off this backup feature.
To do so, open the Dialog box by hitting the Windows and R keys and then enter the command cleanmgr. Click on OK and then Clean up system files followed by another OK.
Make sure there's a tick mark next to Previous Windows installation(s) and then a final OK. This will get rid of the backup, hopefully freeing up space on your drive.