Let me see if I have this correct...

One PC is connected via its network card to the router.

The other PC is connected via its network card to the router, and via its modem to your ISP, and has Windows' Internet Connection Sharing set up on it.

Correct?

If so, in order to use this configuration, I think you would have to configure the router to assign IP addresses to its internal interfaces as a DHCP client - your machine running Internet Connection Sharing is a DHCP server, your other machine has to have the default gateway for its network card set to the IP address of the interface of the router that it's connected to, which you wouldn't know until the router's started up and has been assigned addresses for its interfaces - if you can even configure it that way. There may even be more than that to do - it's hard to say without actually being there. My suggestion would be, as I believe others have given, to replace the router for now with a switch or hub (switches are better) - it will make life much simpler. Or, connect one PC to the other using a crossover cable.

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