ok, first things first, what OS are you using?
I just plugged my modem in, XP said "oooh theres something new better sort myself out" and then came back and said I had new hardware.
On the disk that came with my install there was something to remove the USB drivers. might be an idea to do that. Remove the modem. Reboot the computer. Shut down. Then plug the modem in and see if it comes up as new hardware.
You may need to register a new MAC address.