When you update your system from Windows 8 to 8.1, or upgrade from Windows 8.1 to 10, it keeps your previous Windows system files in “Windows.old” folder and the installation files of the new system in “$Windows.~BT” folder. These folders can be used as backup to recover any files or settings that may need after update/upgrade to the new system. Usually, they use lots of disk space on system drive, sometimes, it could takes up to 10 GB or more. If you’ve been used the new system for a while and you’re confident that you don’t need any files or settings from the previous version, you can safely delete these folders to reclaim disk space using Disk Cleanup.
In this video, I will show how to use Disk Cleanup to delete Windows.old and $Windows.~BT folders including other temporary installation files on Windows 8.1.