The windowed mode in this game is crap.
I'm sorry, but it really is. Not only does it move up slightly whenever you alt tab (which it absolutely should NOT do - why have windowed in the first place?), but you can't move it around at all. I believe you used to be able to do this but you can't do it at all in 2.6.
I absolutely cannot play the game in this state. It's broken and it doesn't work. Also, widescreen fullscreen doesn't work properly at ALL. I am a big multitasker and I like (most of) my games to be windowed. I would love to see a fix for this in the next version and/or 3.0.