Hey, welcome to the forum.
Could you first tell me which webcam you have (the make and model number)? and also the operating system (OS) are you using (Windows XP, Vista, 7, etc.)?
The webcam needs a driver
[?] which is compatible with you're OS - it tells your computer how to use the webcam. If it doesn't, it may not work. It could be that you have an old webcam with a new operating system, which could cause problems when using it with Messenger.