Need help with networking

So I'm trying to set up a private network to share files between my laptop and my desktop via LAN while still using WIFI for internet. The WIFI is through the uni so I can only have one one connection at a time but I can share it through my desktop to my laptop. The problem is when I try to set up file sharing on the desktop it won't only share through LAN and tries to share with everyone else on the WIFI network. Was also wondering if there was a way to make it permanent because it's a pain in the ass having to set it up again any time one is disconnected or shut off. Especially when you aren't allowed to change settings yourself and are forced to the wizards and troubleshooters. 

P.S. sorry for posting this here but after days in networking forum I'm just going to assume people still don't go there.

Oh win7 desktop and win 8 laptop BTW.

I would recommend building a basic file server. If you have the extra money. I don't do the desktop sharing for that simple fact. If you have an extra computer laying around, turn that into a NAS using FreeNAS

Umm connect to wifi via your desktop and get a crossover cable between your laptop and desktop then. Should do the trick. Be sure to set up the IP addresses for the crossover as static on another subnet