The overscan would be for the PC input, in most cases. Since you're using HDMI, the picture on the scree is displayed exactly as it is sent (with a pixel/pixel representation).
However, if there is an overscan, it would be at the source, meaning in the settings of the PC or graphics card. Try to update your graphics drivers or turn overscan in the PC down to 0%.
The PC setting is grayed out because you're using the HDMI port, and not the 15-pin D-SUB.
the problem i am having is when connecting my TV to my PC via an HDMI cable in the HDMI 3 port i am able to get the image to display however the image extends beyond the edges of the TV i believe this is called overscanning(not sure on that) i try to use other resolutions controlled by my pc and get the same result. i have read my manual and it tells my to goto menu>setup>pc and use the auto adjust option however the pc setting is grayed out an i am unable to select it. please help me.