A Button camera is really small and can be used as a webcam
http://www.ebay.co.uk/itm/Mini-Button-Spycam-DVR-Camera-WebCam-Voice-Video-Audio-Recorder-Camcorder-30fps-/271070262988?pt=UK_Gadgets&hash=item3f1d0bf6cc
Not sure how well it would work in a dark room, you may need to add some extra IR lights for that.
If you want to process video then you need something like a Raspberry Pi.