Network/Internet data is sent in packets of some size determined by your computer and the server. In other words, you recieve blocks of data at a time (7kb blocks for me). Now, if one of those packets is screwed up, interesting things happen. Obviously, there's a good chance you lose or corrupt data as it moves through your cord. That explains why XP is screwy. Now then, most routers will only draw as much electricity as they need. If only one computer is talking to it, it won't draw as much power. But, if it thinks another computer is connected or trying to connect, it will draw more power. It's a long shot, but I think when you have both computers on and only one really connected you get a better data transfer on the wire, and don't lose any packet information. I'm not completely sure without getting the probes on the router, but in theory it could happen.