How did the machine connect to the interweb before this issue?
Have you tried other USB ports when plugging in the modem?
[1st test]
Shut down the machine.
Unlplug the MODEM from whatever USB port you have it in.
Power on the machine.
Plug the modem into ANY usb port.
What happens? Does it say "New Hardware Found"?
Document everything that happens. ex: notices? prompts for installs? anything would help.