Restore playlists from Windows backup

The playlists are all part of the library; you'll find those files all in:

C:\Users\<yourusername>\AppData\Roaming\Pioneer\rekordbox6 -- note that the AppData folder is hidden; enable hidden files in the Explorer preferences.

You can copy the entire folder over and it will be able to load provided the username is the same on both computers, otherwise you'll need to edit the rekordbox3.settings file within.