So you have your modem connected to port 1 with a CAT5 cable, then your computer connected to port 2 with a CAT5 cable, and your xbox connected to port 3 with a CAT5 cable? The first thing I would do is make sure the cable is plugged in all the way at both ends, then I would check the link status light. If its not blinking plug it into another port and try again. If that doesn't work try another CAT5 cable. If that doesn't work unplug your PC and try to connect to XBL. And if that doesn't work connect your xbox directly to your modem.