I would have to suspect that the drivers are corrupt...or the networking device is bad. It could be a Windows problem, but I would not suspect XP at this point.
I would uninstall all network devices in Device Manager, then reboot and reinstall same with fresh or new drivers (ensuring that they are the correct drivers for the device).
If no joy...I would try temporarily installing a second NIC, to see if a connection can be properly made. It would not matter whether this NIC is wired or wireless, although I think wired is easier/quicker.
I see that you have a laptop, forget the suggestion about the second NIC (made with desktops in mind).
Edited by hamluis, 03 February 2009 - 12:24 PM.