Windowed mode?

At the time no. There is windowed menu mode in the options that lets you use a windowed menu as it states but when the game starts it will go to fullscreen.

There is a work around however. When the game starts alt-tab out (or ctrl+alt+del if you have nothing open to alt tab to) and go to you screen properties and set your resolution back. The game should be windowed in the middle of the screen.

Note that this is a rather buggy way of doing it and is not intended, so if it does not work out to well then your out of options.