20mm gap around all the screen edges

hi, dont suppose anyone can help. ive just got a inspiron 560 with 23" hd lcd dell monitor, all stock items from Dell. i have been using the AVG connections for my monitor, but decided to get the HDMI Lead. when i connnected that up i noticed that the desktop no longer fits the screen fully as it did with the VGA. there is about a 20mm gap around all the edges. also i did't really notice a signifignant improvement in picture quality! strange.
but my main goal is to somehow get the desktop to fully cover my screen, any ideas?
i've gone through the steps of screen resolution, and have selected the optimal settings offers (1920x1080). it did not offer me any larger options, and maybe i need 1920x1280? thanks for any help

i think you might have a 1920x1200 monitor... is there a way to find the model number and post it here?

it should only have black edges on the top and bottom though if thats the case, its weird that they are on all 4 sides

If you have an Nvidia graphics card, see if the "Adjust Desktop Size And Position" feature in the Nvidia Control Panel helps. Something of this sort should be available with other graphics cards, but I don't know.

assuming you picked up this monitor

DELL Display : 23in ST2320L UK Full HD WLED Widescreen Monitor (VGA, DVI-D and HDMI) : Monitors: Flat Screen LCD Computer Monitors and Displays | Dell UK

native resolution is 1920x1080 which should be available from your display options, if your still getting the border with that resolution check your graphics card manager ati catalyst or the nvidia equivelent for a scaling option with in the display options.

no idea which graphics card you have though i can't find an option to pick one for the machine so maybe you added one after?