I have an ExpressCard HP HDTV tuner card that gets HDTV from cable, digital cable, or antenna. Since we have Dish Network, I'm only able to pick up a few channels with the antenna, but I got it to input from game consoles anyways.
However, I don't think you are looking for a tuner. You're wanting to watch PC on the TV, not TV on the PC, right? If that's the case, make sure your laptop doesn't already have TV-out capability (many do). Modern laptops often have an S-video connector that has 3 extra pins to support composite (yellow plug), S-video, and component (RGB) outputs from one port. Most laptops also have a VGA output to hook up a computer monitor. Some laptops also have an HDMI or DVI port (my new dv9700t has HDMI, VGA, and 7 pin S-video, HDMI can also be used as DVI using a special adapter cable).
Most new HDTV's have HDMI, VGA, and Component inputs that support HD resolutions. Remember that HDMI ports on your TV can also be used with DVI ports on your PC using a special cable.