Webcam security - can people hack in my webcam even though I am not in MSN, etc.?

I have a built-in webcam in my notebook (which I never used before, dunno whether running or not). So whenever I am doing my work on the notebook or browsing the Net, can people actually hack into my webcam and spy on me? I understand that it is possible that people hack into others' PC or webcam, but if I do not turn it on (I dun know how to turn it on anyway), can they still hack into my webcam and spy? Pls advise.