Just wondering, why are you using the USB connection? The modem settings / changes should be do-able via the ethernet connection, usually by typing in the modems IP address in an Internet browser window - this IP address should be available in the documentation supplied with the modem.
You can also get the modem IP by going to Start, Run & typing cmd then in the window that opens up type ipconfig /all. Somewhere in the resulting list there should be a default gateway listed - this is of course assuming the modem is connected via the ethernet port and Vista works the same as XP for the above commands!
I hope this helps, but this could also be total BS if you have a modem with no ethernet port.