If you've upgraded from Windows 7 or Windows 8 and you find that a lot of your harddrive space has been used up, don't worry too much as it's not Windows 10 itself that's packed it out.
When you upgrade, Windows keeps a backup copy of your previous OS to allow you to revert back to it if you're not happy with Windows 10 - however this is only valid for up to a month after you upgrade. Now this backup doesn't exactly come in at a small size as you can see below.
That's (for me) 22GB of useless unneeded data that can be removed ONLY if you're happy to stick with Windows 10. This data is stored in the following three locations:
If you do not see these folders, then when you open Windows Explorer. At the top click FILE and select "Change folder and search options", once the box opens, click the "View" tab and in the "Advanced settings:" section, find "Files and Folders -> Hidden files and folders" and choose "Show hidden files, folders and drives". Click Apply, then OK and now you should see them.
Are they safe to remove? Yes they are, but ONLY IF you are NOT wanting to downgrade back to Windows 7 or Windows 8.
** NOTE ** You are doing this at your own risk and I can not be held liable if anything messes up. Proceed at your own risk
If you are more than happy with Windows 10 and want to free up some more space on your harddrive, then go ahead and delete them three folders. This task will take quite a while when using the native right-click and select Delete option in Windows Explorer, so using a handy program like CCleaner will do it much quicker and easily for you by selecting the Advanced -> Old Windows Installation option.
Or use the built-in Disk Clean-up tool (Windows Key + R, then type cleanmgr).
With the Disk Clean-up tool you will have to select the "Clean up system files" button when it loads up and then you'll see the options (which is unchecked by default) "Previous Windows installation(s)" and "Temporary Windows installation files" - these two need to be checked.
One thing to note though, is that the Disk Clean-up and also CCleaner will only remove the $Windows.~BT and Windows.old folders. You will have to remove the $Windows.~WS folder manually.
Once they're all deleted, you should find you have a lot more space on your harddrive. :)
And finally, if you're wondering what these folders are, then from what I can see:
Windows 10 installation boot setup files/logs
Windows 10 setup files (like an extracted ISO)
Some of your old system files and some program files that can be retrieved if upgrading didn't transfer everything