"I do not see any window."
Translation:Non vedo nessuna finestra.
Niente is translated to nothing in English. So, when this happens, you are basically saying, "non vedo niente finestra" ,which translates to "I don't see nothing window". This is technically grammatically incorrect.
So, by doing this, you imply, "nessuna=any", for the response. Thus, making this sentence grammatically correct.
Qualsiasi only translates as "any" in a positive sense. You may like to check my discovery of this on Duolingo at http://duolingo.com/#/comment/231470?from_email=comment