This has been reported by two of the other people that use this PC. One of them, who plays the game Hearts a lot, says that twice when she has started the game, the webcam software has started up as well. The other says that once when he started Firefox the webcam software started up as well. So it doesn't happen reproducibly for either of them, and it hasn't happened to me or the fourth user at all (yet). I'm struggling to come up with a paranoia-free explanation, but MBAM and Avast! both find nothing amiss. Does anyone have any suggestions to offer?

It's possible that for some unknown reason the software is being triggered by applications that access the web (as both of those programs would), but only sometimes. As for a true explanation, I've got none .