Windowed mode???

[The Gooey Fox™]

I am putting fullscreen=false into the DungeonSiege.ini but it stays in fullscreen! Please help I have no idea what to do. Whenever I play the game and alt-tab the game will not open because of my 2 monitor setup.

[furbleburble]

Remove that from the .ini. Instead, try this:
-Right click the game in your library
-Click properties
-Click "Set Launch Options"
-In the little box that pops up, put "fullscreen=false" there, without the quotes, then click ok
Should now do what you want. You might run into screen tearing though, or other issues.