As the title says and you can see in the image: The monitor is connected through HDMI to my laptop. My screen settings are as you can see on the image, full res on both displays (1080p on external, 1366x768 on laptop) and "expand desktop to both displays". I have full 1080p res on the external display, but it gets scaled down on the monitor and there is an ugly black margin (marked red in the included picture). It works without this problem on linux on the same machine. Laptop is a Sony Vaio VGN-NW21SF, with a ATI Mobility™ Radeon® HD 4570.
Thanks in advance.