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Personnaly I'd be suspicious of the USB network connections. Unfortunatley USB means the PC itself could have to devote a lot of resources to the working out what the network is doing / wants, whereas dedicated ethernet cards have their own hardware on board to do that, letting the PC get on with bigger and better things. However that doesn't fully explain your situation.
How were you connected before to your modem, when you didn't have 2 PCs, was that USB or ethernet?
What are you trying to transfer. Make sure that you are not saturating the upload bandwidth with too many signals.