Try your monitor's Menu button while in the game. There's probably an Info menu that tells you what resolution the PC is sending.
Assuming that's 1024x640, you need to choose how the monitor deals with resolutions smaller than its native resolution. When switching to a new resolution, old CRT monitors used to make it fill the screen, but LCD monitors often show it pixel-for-pixel with the rest of the pixels black. On my BenQ G2220HD, I choose Menu | Picture Advanced | Display Mode | Full.
You may also have a similar setting in your graphics card's driver (often on the task bar, or in Screen Resolution | Advanced Settings). On my Intel i3, it's Advanced Mode | Display | General Settings | Scaling | Scale Full Screen (or Maintain Aspect Ratio, if you choose a resolution at a different aspect ratio).