If all you want is the TV to display the computer's display like a monitor does.
Then yes and no.
If your video card has a TV Out, this is usually a S-Video connection, then Yes you can do it easily.
If you're using on-board video, which usually don't have a TV out port, then No you can't.
A TV Tuner card would be used if you want to capture TV to your computer and there for use your computer to watch or record TV.