I don't think you would be happy to connect the desktop to a TV-set, unless you only use the PC for viewing movies. The resolution of a TV-screen is far less and really unworkable for anything else than that. If it doesn't work with a new computer, go back to the shop and let them explain how to do it and show that it works. It might be you need to do some setup in the video driver to get the TV-out working.
I never saw a laptop with the option with an incoming connection, so you can use its LCD-screen as monitor for another computer. Usually they do have an outgoing connection to hook up an external monitor, but that's it. You'd better read your laptop's user manual to see it's possibilities.
Hope this helps.
Just baught a new computer but didn't get a monitor, i have the cord to set it up to my tv but it isn't working, i was told i could set it up to my labtop and see the desktop on the labtop monitor, how do i do this?