Install windows via a bootable flash drive or disc. After booting up from the disc get to the drive menu and :
Choose the Custom Option on the same partition where the old windows was installed. It will display a dialog box asking you to rename the previous windows folder to windows.old where every folder in C: is backed up
So It'll rename the previous windows to windows.old where the previous yet unusable windows is stored.
You can still access all your files/folders in that windows.old directory which contains the copy of your previous folders from C: like the Users, Program Files, Intel folder, AMD, Programdata, Program files etc!