You probably need to right-click on your desktop and go to Properties-Settings-Advanced (or go to Display inside of your Control Panel). Inside there, depending on your video card, you should be able to enable your TV out. If you do you might want to set your resolution to 640x480 (maybe 800x600, not sure) before doing so just in case. If it's set too high sometimes your TV won't be able to accept the signal.
Hope that helps.