You have the router connected to NIC1, and NIC2 has nothing connected, right?
Then just get a crossover cable, and another NIC to go into the second computer and using a crossover cable connect them. Run XP's networking wizard and your done.
If you need connections for more than 1 extra computer it would be best if you buy an ethernet switch and a NIC for each extra computer and connect them to the switch with one of the connections in the switch going to the primary computer with the 2 NICs.