Well If you want to use Linux + Windows, the best practice is install Windows first and then Linux. Windows Installation will not detect any existing Linux installation, but the Linux does.
But if Debian is not detecting the Windows, you can manually add the entries in boot configuration and try booting it.
title Windows OS
My Windows OS partition is /dev/sda2, so I added (hd0,1). You want to replace this in your system accordingly.