Need help with my computers wired network connection

I have a DELL INSIRON 5100 with WIN XP, and a Belkin cable modem with a U.S. Robotics 802.11g Wireless Router. My laptop works through the wireles connection, but when it is plugge in the the routers LAN connection the Limited or no connectivity bubble pops up. Other laptops work with the wireless and the plug in LAN connection. When you plug my laptop into the modem directly the same limited or no conectivity bubble shows. If you unplug and plug in the modem it will connect for about 30 seconds the disconnect again untill unpluging and plugging backing in but will not stay connected.