i think the best way to fully remove the modem is to load in to safe mode by hitting f8 a few times when the system starts

then go into the control panel

look for system click it

click on hardware tab

then device manager

then look for modem in the box

expand it and it should say us robotics ect...

right click the modem

choose uninstall

and then shut down system

take old modem out install new one and drivers

hope that helps :)