Press ctrl - alt - del for the task manager and see if you can spot a program that might be trying to access the internet for some reason. Also check the quick start tray for a hidden icon that is trying to access the internet. Also check on your ME machine as it could be trying to connect also and using the XP machine to do so.
I'm just guessing here, could it be caused by the way you set up the network. Since i dont know anything about networks, I advise you to have a look at the way you set it up again, and check that you didnt involve in anyway the 56k modems on either machine.
Hope that gives some help
