The box is not set up correctly. This is typical of Comcast installers.
First, confirm that the box is connected with either component video cables (red, green, blue, white, red) or HDMI. If not, that's the first thing that needs to change. Composite (yellow, red, white) or coax will not provide a high definition picture.
Next, turn the box off, press the menu button on the Comcast remote, change the YCbCr output to 1080i, the 480 override to 480i, and the aspcect ratio to 16:9.