Tools > Preferences > Show settings = Simple > Video: Output [DirectX (DirectDraw) video output] << it is also Default or Automatic module on Windows XP.
If I play a film in one instance of VLC and then open the same (or some other) film in another instance, then the picture in the second instance of VLC is brighter (or the first instance has darker picture than it should). Do you experience the same problem or is it just my crappy computer, outdated vga drivers (GeForce FX 5200)?