Hi,
The game is from Steam. Fallout 3 GOTY with lots of mods.
I have a 3 monitor setup. I was looking for a way to "seamlessly" and comfortably use my other 2 monitors for web browsing, etc, while playing Fallout3 on the "main" monitor. I found this solution: http://www.actualtools.com/multiplemonitors/how_to_play_starcraft_2_on_dual_monitors_tips_and_tricks.shtml.
I installed the program, tried it on Fallout 3 and it won't work:
Running the game in fullscreen mode: the mouse does NOT leave the monitor, it is stuck in a game monitor and I cannot move it to either of my other 2 monitors. Alt-Tabing minimizes the game (which I was looking to avoid in the first place).
Running the game in windowed mode: same thing, mouse does not leave the game window. I tried Alt-Tabing: game does not minimize (good), but the mouse is "gone". When I alt-tab from the game, the keyboard switches to the active application, but mouse stays in the game window! wth?
I tried running the game with and without the "Fake Fullscreen Mode Windowed - Alt Tab Fix" mod launcher. Same results.

Is this Fallout3 fault? Does Fallout3 "lock" the mouse or something?
Btw, I tried running Portal 2 with AWM - works perfectly. When not "in game" and mouse is "loose", I can move mouse on all 3 of my monitors. So it kinda looks that Fallout3 is to blame...


Edit: -------------------------- solution:

About an hour after posting this, I tried something and it worked! it seems Fallout3 was "locking" the mouse. I went into Fallout.ini and changed bBackground Mouse=0 to 1. Yippee-ki-yay.