Dungeon Siege Windowed Mode __FULL__
As for desktop resolution, mine is also 19201080 and I find the 1600900 resolution to work quite well for the game in windowed mode (Of course, I maximize the window to cover as much of the display as I can).
Dungeon Siege Windowed Mode
Expanding a bit on that, if you want a borderless fullscreen set your desktop resolution to the game resolution before launching it and make sure to check "Disable Alt-Enter to toggle screen state" in dgVoodoo2 settings. On my computer that makes dgVoodoo2 behave weirdly in window mode. Also obviously disable "Application controlled fullscreen/windowed state". 350c69d7ab