Internet Connection problem

Hello everyone, I have a problem which is very confusing, maybe someone can shed some light? I have a problem connecting to the internet, I have a Broadcom Xtreme network card in my computer which connects to a webstar dpx100 cable modem via an ethernet cable. Recently the local area connection says I have limited or no conectivity, I have checked the cable and it is fine. The local area connection looks as if it is connected but I have to use the usb connection on the modem to connect to the net. Does anyone know why this would happen?

Simon