Network Problems?

Virgin Broadband to wireless router.Desktop xbox all hard wired to router,all wireless devices working ok.Got a new desktop in another,so hard wired a cat5 to the room from the router,stuck the lap top on the cat5,connects all ok.The problem comes when I connect the cat5 from the router to the new desktop;it sees the connection but will not connect to the internet.If I take the cat5 out from the router and plug directly to the modem[to test] all works ok...any ideas welcome from the world.