When you install or reinstall Windows without formatting the drive, Windows setup moves the existing Windows installation to a new folder called Windows.old, meaning you don’t need to backup all data before installing Windows.
In other words, the drive containing existing Windows installation must have a minimum of 16 GB for 32-bit and 20 GB for 64-bit free space in order to perform a fresh installation of Windows without erasing the drive. The fact is we can install or reinstall Windows 7, Windows 8/8.1 or Windows 10 operating system without formatting or erasing Windows drive provided that the drive has plenty of free space to accommodate the new installation.
