Textures mapped to window coordinates getting blured


from what i understand, the Window mapping mode should output the image texture within the render window undistorted (meaning: using no further 3d information).

However, when i use a texture like this, the output gets blured:
The top part of the above image is a background image mixed in with the compositor. The bottom part is the exact same image on a simple shadeless plane, mapped to window coordinates. The image has the exact same size as the render window.

Am i missing something here?
Is there a way to avoid the blur?

Any thoughts are appreciated.
Thanks in advance