a network device is turning off during sleep or restart
when the main machine goes into sleep mode, or is restarting, the other devices on the network (running xp) say "a network cable is unplugged." i have a feeling that the primary machine is somehow powering down the modem or router. i have unchecked the box "allow computer to turn off the device to save power" in the power management settings for the network adapter.
Try updating the NIC driver on your PC, and seeing if that makes a difference.
Set your connection as "nailed up" in the router.
If your hardware has automatic cable recognition it'd pay to ensure you are not inadvertently using a crossover LAN cable. It would pay to try a fresh cable anyway.
You could also check your DNS servers are not set differently in the adapter and the router.