This is fairly easy to do. Just go to your local computer/electronics store and ask for a crossover ethernet cable.
Connect the 2 ethernet cards using the cable. The ethernet card on the main computer is showing it as disabled/inactive because there is no cable connected with anything on the other end of it.
Set up the IPs on each computer for their cards. For example, you could use 192.168.0.1 with a subnet mask of 255.255.255.0 for one, and 192.168.0.2 with the same subnet mask for the other. That would alllow interconnectivity between the 2.
Once that is done, make sure that the user accounts on either computer have passwords. Then, make sure that they are in the same usergroup (right-click My Computer>Click Computer Name tab).
To potentially save some typing, do you know how to share files? And, does the above work so far for you?