It sounds like you have one crossed cable and one straight one. Usually its straight cables from a modem or router to a pc but a crossed one if its from pc to pc.
Most routers these days have auto sensing and will actually work on a crossed cable as well as on a straight one.
Does the one that doesnt work have a colured band on it or a label that says its crossed ?
Its probably just a case of getting another straight ethernet cable to connect the second pc/x box at the same time