You might not realize it, but whichever operating system you upgraded to Windows 10 from (assuming you didn't install the OS from scratch) still sits dormant on your PC for a while. This will, of course, reduce the amount of storage space available on your system drive.
To fix this, type "cleanup" into the Cortana/search box (next to the Start menu) and choose the Disk Cleanup utility from the results. When it finally appears after scanning your drive, look for a file called ‘Previous Windows Installation(s)' in the list of files you can clean up. Make sure this is checked, then click OK. Windows 10 will then delete the leftover files from the previous operating system, freeing up 5-10GB of space.
If your PC has more than one drive, you'll have to point Windows to the system drive where the previous OS was installed when you run Disk Cleanup (usually C:).