Did the USB ADSL 'modem' feature an RJ-45 port for use with a network interface card? I've seen a few UK ISPs offering USB 'modems' which feature both USB and RJ-45 based methods of connection to a computer.I've always found that the most reliable way to connect to a broadband service is using a good PCI network card connected to a 'modem' which connects to the broadband line. I know a couple of people who have connected to broadband cable connections using USB who had some serious problems as a result - problems relating to having to communicate with the modem using unsupported Windows drivers and problems with the device demanding too much power from the USB port, sometimes resulting in the system shutting itself down or in other USB ports shutting down.
If you want to share your broadband connection with several systems, my advice would be to build a cheap server (with two network cards - one connecting to broadband and one to the local network) and network all the computers you need to specifically for this purpose. However, this will obviously cost you in terms of both time and money but you do get the added advantage of having a machine dedicated to sharing the connection and you could even set-up a firewall on this system.
-Stephen.