I have an XP computer connected to the internet via a USB broadband cable. I then have a second

win 98 computer connected to the first via standard network cable using the NIC's on both machines. I've used the XP installation CD to set up the home network, but I can't get the internet connection shared, even though windows detects and displays an internet gateway.
Is it possible to share an internet connection in this way, or will the ISP detect the second MAC address and prevent access, thereby forcing the purchase of a hub (USB or ethernet)?