if you are using the webcam on yahoo and want to also use on msn.. it wont be possible. you can use webcam once at any chat client. so better close yahoo webcam and then try it on msn or skype.
As Aaho said, shutting off Yahoo should solve the issue. Usually when you "close" those IM apps they don't really close they just get minimized into the system tray (where you see the time).
And as sheyn said, if you restart the computer it fixes the problem also (in essense the IM application gets nuked during restart).
Webcam is not being used simultaneously on all three applications. Without Skype or Yahoo messenger running, MSN does not recognize webcam is not being utilized by any other program and SKYPE simply doesn't detect that there is a webcam. Same is the case not only on the desktop computer but also on the laptop.
^^ try reinstalling drivers. if it also doesnt solve the problem then go to the preferences / options/settings of the chat software and then go to webcam option in it and seeif there your webcm is selected / active. If it is not active there then you have to select it as per software options.