I have a Lifecam Cinema webcam and it won't let you use anything except the built-in mic if you're using the Microsoft program. There's other programs out there that capture live video from webcam and then you use that file to edit it in whatever program you want but their names escape me for the moment. Maybe Camtastia Studio. You can try the free trial and see if it's any good. I only use the screen recorder and not the actual Studio part so I'm not sure.
http://www.techsmith.com/camtasia/features/win/