If you carry out a clean install without formatting your hard drive you may be left with a Windows.old folder, but I wouldn't guarantee it.
I stand by my previous advice that a backup of your personal data to an external hard drive is best, and by that I mean physically copying the data over so as to avoid any complications with Windows Backup and Backup Restore.
It's better to take a bit more time and preserve your precious data rather than rush the job and lose it. If you have a lot of data to move, let your computer do it overnight when you're asleep.
However, if you do finish up with a Windows.old folder, this tutorial explains how to remove it to regain hard drive space. Windows.old Folder - Delete