no one can see webcam

Hi
I have recently bought a webcam but could not get it to work in messenger
i could see cam working here but everyone was getting black screen and could
not see.
So i purchased another webcam yesterday and same thing is happening the
webcam is Vista compatiable ! I dont know why no one can see ! I click on
view my webcam and nothing !! What am i doing wrong? or can anyone help
solve this problem i am having
Thanks