Importing a slow-mo video to Windows
The slow-motion effect applied to the video in Photos.app is not actually applied to the video file. This means that the 120 fps video remains at 120 fps and is played at this higher fps in Windows.

To slow down the video, use a video editor, either on Windows or on the iOS device (such as iMovie which is free on the 5s) to save the slowed video file.