You can also look at your computer's logs to see when it's been turned on/off etc, and possibly give some idea of what it was used for. Won't tell you who the person is, but knowing when they do it could help narrow down the possibilities (eg. if it happens while your housemate is at lectures, it can't be them). In Windows 7, go to control panel > administrative tools > event viewer. You'll probably mostly be interested in log entries created by "Microsoft Windows security auditing", which can mainly be found under Windows Logs (on the left) > Security.
Or, to leave your webcam recording, just use any old webcam recording software, and leave it running while you're away. Analysing several hours of footage to see if anyone appears in it would be a pain though, and that's assuming your laptop has the space for several hours of video (and the culprit doesn't see it recording and delete the video file). I can't think of any software that can do it secretly, so it might be your best option (other than the keylogger/system logs).
