Michael has a computer that drops off the network from time to time, causing him to restart either the computer or the router. Leo says it's probably not the router since other computers aren't effected. If a computer drops off the network, then there's probably an issue with Mike's ethernet card since it's wired in. He should first replace the ethernet cable. That's the cheap and easy fix.
If the problem persists, then it could be the network interface. Michael should go into the Network Control Panel, remove it and reboot. Windows will reacquire it and could fix the problem. He should also try updating the drivers, and make sure he's allowing the router to set the IP automatically.
The Chatroom says that if rebooting it will work, then try clicking Start > Run, then type IPConfig /release, then IPconfig /renew from the command line. That could clear out the IP and reconnect it. Also, he should try unplugging the modem and the router and starting over. If that doesn't work it may be a hardware issue and since it's on the motherboard, that would require a repair.