Have a usb flash drive or drive in an external enclosure? Backing your files up from the laptop to transfer over to the desktop and then having WMP there perform an all media search during the initial setup as well as creating the new playlists will take care of the playback issues immediately.
As for the network situation if you have a router coming off of the dsl or cable modem the laptop connects by way of the guest network while the host pc connects through the host network configured to the ISP and router. If the router has a firewall you would have to set that up with password protestion and then may have had to create a usb set up key with a flash drive to see the laptop added on in order to share the connection.
The other option for file transfers between two machines you could opt for would ne the TeamViewer program where the two use the remote desktop connection to get sterted and then you would use the file transfer wizard which is a bit more time consuming however. TeamViewer
You'll see where DocBrown pointed out the file sharing capability of the program in the thread seen there. Recently I had quite a successful review of the program. In fact you can see how that looked here when accessing the main Ultimate case here from the second 7 Pro case I put together lately from spare parts? as well as seeing onto the 7 Pro case from the main.