For reasons you can't mention?
Logmein is a great site, I use it a lot. Let's say your at work on your break of course and want to see whats going on at your house. Use logmein to sign into your laptop. Fire up whatever program your using to view your webcam, I already mentioned irfanview. The webcam view should be viewable on your work computer, showing you whatever you pointed the laptop at. (assuming the cat didn't knock it over already.)
If you are hoping to turn on someone's cam without them knowing, can't really help you there. There are ways that I know of, but not on this forum.