If you're on a budget and can hide a cable pretty well, your best bet is to purchase or make a cat5 crossover cable and connect the two computers directly. The cable modem should have a usb that you can use to plug to your first computer since this computer will have its nic used up by the crossover cable. You will have to enable internet connection sharing on the first computer. Assign a static ip address on the second computer so that it may be on the same network as the first computer. Windows by default assigns the ip address 192.168.0.1 to the nic on the computer that you enable ICS. Make the address of the second computer 192.168.0.2 255.255.255.0.
They will both have internet access, but you won't be able to enable file/printer sharing between the two computers.
This is assuming you have no router with a four port switch or a switch alone.
If you do, you can enable file/print sharing and you won't have to enable ICS.