I am assuming you are connecting the computers together directly via their network connectors and a cable. Is the cable a "Crossover" cable?
If it's a cable manufactured by one of the larger companies, it will normally be marked on the cable. It can also be identified, by looking at the plug ends, using the same orientation, and two of the wires (colours) should be reversed. If they are exactly the same, you have a patch cable.
In this instance, a "straight through" (patch) cable will not work.