MojoKid writes: Microsoft may have given you a free Windows 10 upgrade but it's not without some left over file clutter that some folks don't realize is left on a system after migration. It's not rocket science but there are a few key strategies to tidy up the file bloat an OS migration can sometimes leave behind and to further optimize an upgraded Windows 10 installation. The ability to roll back to your Windows 7 or 8.1 installation within 30 days is a very easily overlooked feature of the Windows 10 upgrade process. The feature is a lifesaver if you encounter issues, such as missing or incompatible drivers, and need to quickly recover without starting from scratch. This ability doesn't come without consequences, though. In order to offer this feature, Windows 10 is essentially keeping another completely separate Windows installation on your PC. This will need to go, once you've determined that you are sticking with Windows 10 and everything seems happy enough. These files are scattered throughout your system and include a number of hidden directories, with the bulk of them located in Windows.old and the hidden $Windows.~BT and $Windows.~WS directories.