Typically, a clean install of Windows 7 means removing the existing operating system (such as Windows XP, Linux, Windows 7, Windows 10, Windows 8, … it doesn't matter) and replacing it with a new or "clean" installation of Windows 7.
Format and Clean Install Windows 7 [Tutorial]
In other words, it is the process of “wiping everything and starting over” for Windows 7, a procedure also called a “clean install” or sometimes a “custom install.” It is the ultimate process of “reinstalling Windows 7.”
A clean install is often the best way to fix serious Windows 7 problems, such as a virus infection that you can't completely get rid of or another Windows problem that you can't fix with normal troubleshooting.
Performing a clean install of Windows 7 is also usually a better idea than upgrading from an older version of Windows. Because a clean install is a true start from scratch, you don't run the risk of carrying over buggy situations from your previous installation.
