Tethering not work with active LAN connection iPhone 4

I have a simple setup with two computers, PC1 and PC2, connected through a router. I want to use iPhone 4 tethering to give internet access to PC1 while at the same time accessing files on the network (PC2), but this setup does not work. Let me explain.
Simply enabling tethering on the iPhone yields a total of two connections in the Network and Sharing Center (Windows 7), iPhone4 connection and LAN (physically connected to the router), where the iPhone4 connection shows that it has internet access. Trying to use internet, web, ftp, or whatever fails however. If I disable the LAN connection so I have a single connection (the one to the iPhone) I will get internet access but ofcourse I then loose access to my LAN and PC2.
This can not be by design, my setup should be more than usual, yet I have seen noone else post on this problem (using a simple google search). I use the latest iTunes (10.1.1.4) and iOS (4.2.1) and my provider is Halebop (Sweden).
iPhone4 + Custom PC Windows 7