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You cannot communicate 2 PCs using USBs directly. USB works in kind of master slave mode. However, there are some USB cables that have a circuit in the middle. Both PCs see this circuit as a USB slave and can thus communicate with each other.
even if you design this circuit, you will have to make your ow interface protocol similar to ethernet anyway
keep in mind how much programming that would be
i suppose you want it as a driver, so if thats the case youd need to get your hands on windows ddk (winxp sp1 edition is free, but you have to pay $15 for shipping)
in the end its more worth it using a virtual ethernet port over usb (which there is software for out there somewhere) and purchase a the cable for it