[quote name='flowery']It is plugged directly into the DSL modem. It is an integrated Ethernet adapter.[/QUOTE]
It might be the modem, or might be the Ethernet adapter. Reinstalling the drivers for the Ethernet card might help, or installing newer drivers. Which drivers would depend on which card it is. If you open up the Network Connections control panel (assuming it's XP), it'll show what type of card it is on the right, under the Device Name, if you view the window under Details view.