5100: "network cable not connected"... but yes it is

Hi The Community,

I got a little problem with my Inspiron 5100. This problem disables me going through the network so it is a little bit frustrating...

When I connect my computer to the network (with a RJ45 cable), the cable is not detected in Windows XP. And on the other side (what else is the network -- i tried with different ones), my laptop is not detected too.

Sometimes, it 'works' : the cable is detected. But it gives me a strange IP or a 0.0.0.0 IP! And still nothing on the other side of the network.

- I've Symantec Antivirus installed with last update and have made an analyzis but no virus was found.
- I can ping 127.0.0.1: it works.
- my network card (Broadcom 440x) is ok in WinXP.

I called Dell Support but they can't help me. They said i had to test it on different networks ("perhaps cable is in cause"). But cable is ok, i tested it with other computers.

Anyone knowing what i could do?