Do you connect your PC via the VGA port or the HDMI port?
Try a TV-supported square pixel 16:9 resolution on your PC (check your user guide if you have it with you). 1280x720 or 1360x768/1365x768/1366x768 usually works. The TV will typically compensate for it automatically, as it is designed to accept square pixel information. You may have to play with overscan settings later on for proper desktop coverage, however.