There is no setting that will make it run in Windowed mode. I have tried to find a switch (-w, -window, -windowed) and they don't work. This game does not have what you want. Unless you know of a third party application to force it, the only method I know of is to run the game within a virtual machine which would pretty much destroy your graphics.
I have the same problem that you do. I use multiple monitors but I cannot use the second monitor without alt - tabbing out of it. I even have a touchscreen with a media center running on it.
Even if there was a way, would it be worth it. Windowed mode takes more out the graphics so you would take a huge hit there.