cheapest way to do it is, get 2 NICs in the main comp and enable ICS on the main NIC and it will give the other NIC an ip like 192.168.0.1 and then go to the other PC and give that NIC an IP 192.168.0.3 and that's pretty much it.
Make sure on the 2nd comp, the gteway is 192.168.0.1