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I'd persevere with getting the ethernet working, rather than the USB. That is unless you beleive the network card in your laptop is faulty.
Start by powering off the modem, disconnecting your laptop from any network cables and reboot the lappy into windows safe mode (F7 during the startup, before the first windows logo). Now go into control panel > Performance & Maintenance (windows XP) > system > hardware > device manager. right click and delete any networking connections.
Connect the ethernet cable to the laptop and reboot it normally. The laptop should now find the drivers it needs (or prompt you for the laptop manufacturer / windows install disk), and once loaded should get you up and running.