Hi Guys,
Just an FYI - You may note there is a Windows.old file in your System drive after you upgrade to Win10. Microsoft will keep this available for about a month just encase you itching to go back to your previous OS.
Now if your not just run the 'Disk Cleanup' tool , then look for and select the 'Clean Up System Files' button, I found close to 20GB being stored here that running Disk cleanup recovered.
Just be sure your happy to stay with Windows 10 as you cannot go back to the previous version of Windows without a full reinstall once these files have been cleaned up.