how to connect 2 pcs to a 4 port DSL 2+ broad band modem

I am using a 4 port modem to access broadband internet. LAN1 port connected to PC (98Se). LAN 2 connected to IPTV. How to connect another PC(Win XP) to LAN3.
I simply connected , one Laptop (winxp) to LAN3. IP conflict error popedup in the Laptop.
Pl.help.