Ah. That makes much more sense.
I'd like to know a definitive answer myself. I have a similar setup on my home computer. Two internet connections on one computer.
I don't know which connection is being used though. The best I can do, is block the ports of things I want to go through one connection in an attempt to force them to go the way I want.
I'm fairly certain that windows xp will set the first NIC that gets an IP when windows starts to be the default connection that everything will try to go through.