If you have a TV out on your system via S-video cable. Attach it all up. Turn the tv on and to the S-video channel and then reboot your computer. Most of those TV outs are not hot attachable, but are detected on post/boot. If it still does not work, look in your display settings and you should see the TV as a second monitor and all you have to do is enable it.

My HD projector is like that via an HDMI cable, if I reboot the box etc with the projector off, the system wont detect it when I turn it on. Has to be on when the system is booted.