Here's the issue with running your computer on a TV.

Most networks do not send out their broadcasts in "true" HD. If it was a full 16:9 aspect ratio, you'd see horizontal black bars on the top and bottom all the time.

To compensate, TV manufacturers include two options for aspect ratios. Setting it to "16:9" actually zooms in on the image slightly, so the TV broadcasts can still fill the entire screen. This presents a problem when connecting a computer through HDMI, because the computer's signal is a true 1920x1080 resolution.

You can fix this by looking at your picture mode and setting it to "Just Scan" (this could be named different depending on your TV model and manufacturer). That should make things look right, but you'll have to switch it back when watching TV.