It is generally better and clearer to use the genitive, although in some cases the meaning doesn't change. In the examples you give, as Hackworth above states, the meaning of the accounting department isn't clear.
In the restaurants of Paris vs Paris restaurants (the isn't correct before the word Paris) the meaning stays the same, so they can be used interchangeably. But the room window implies there is a window somewhere else (e.g. a car) that needs to be distinguished from the one of the room. In the window of the room there is no allusion to another place that contains a window.
The differences in meaning are quite subtle, but they are still there. The safest choice is the use of the genitive.