It was very annoying when I started playing Watch Dogs on my Windows 7 PC and it was always getting started in Windowed mode only except the first time. However there are settings that you can change in the game menu for it, go to display and you'll find options to change display mode to full screen or windowed.
But there is something wrong which triggers to reset it to window mode, so i figured out the following :
If you face this issue where you find Watch Dogs is running only in Windowed mode even after you change settings in-game, then do following:
Or you can also:
If you want to Run Watch Dogs in Windowed mode, then again in <RenderProfile....</RenderProfile> tag find property name WindowMode="2", set it to 0 or 1.