How can I make my sony bravia a 2nd monitor for my laptop?

I have a sony bravia and want to use it as a 2nd monitor for my laptop, I've hooked them up using a serial cable (I think that's what it's called?!) and my TV is saying 'no signal'. Not sure what version of Windows I have as it's a work laptop but it's not Vista, think it's an older one. Can anyone help?|||Most laptops today have something called a Function key (Fn). Usually you can hold down the Fn key and press one of the top buttons on keyboard to enable a hardware function, there might be a little picture on one of the keys of a box or screen. If you hold down the Fn key and press that it should toggle between sending video signal to your LCD, as well as to your external video port (be it DVI or VGA)