Usually speaking, it is a graphics anomaly caused by shelling-out to Windows, in which a player pauses the game, and either hits the <Windows> key, or uses and <Alt><Tab> or <Ctrl><Tab> combination. Another way it can happen is by having a mis-match in your graphics display. If you have a monitor that has a "native" resolution of 1920x1024, set your Windows display to that same 1920x1024 resolution, which should show as "recommended" in the Windows graphics dialog. Once you are in the game next time, use the "Options" menu and "Graphic Display" to set your game to that same setting. Use the drop-down box toward the upper left, and scroll through the list to find a match. If you don't shell-out, and you do have the display set correctly, then are you running any mods? If so, can you list them? Also list your computer specs, OS & graphics card and display.
