Install motion:
sudo apt-get install motion
Enable motion to start at boot:
sudo nano /etc/default/motion
Find the line that says start_motion_daemon=no and change it to start_motion_daemon=yes.
Enable the stream to be viewed from other computers on the local network, and also make the output a little bigger:
sudo nano /etc/motion/motion.conf
Change the following values:
daemon on
width 640
height 480
framerate 100
stream_localhost off
Reboot, and then go to http://localhost:8081 (or the IP address of the PC).