Using TCP/IP for internet and LAN

I have a client that was using TCP/IP for his dial-up internet access and for his internal peer to peer LAN and it worked fine. He had someone else add a PC to his network and he removed TCP/IP for the internal LAN and used IPX. I changed it back to TCP/IP and it works, except now Windows is confused and sometimes tries to connect to the internet via the modem when he accesses something on the LAN instead of using the NIC. I thought I had read something about a registry edit that forces Windows to use the NIC instead of the modem, but now I cannot find the article. Thanks.