NOt sure if this is your setup, but I have done this before:
PC with WIFI connected to router via WIFI accesspoint->switch->router.
Bridged Wifi connection and LAN connection (Different IP ranges). Connected other PC to bridged LAN port with crossover (will work via switch too).
Other PC could use internet.
http://www.pppindia.com/intl/pppshar/ download this program install follow setup and you will never ever have a problem sharing an internet connection again, been using it for a while, simple but it works