DIY "Windex"

General Window Cleaner: using distilled white vinegar and water. Mix equal parts vinegar and water and pour into a spray bottle. Spray the solution onto windows and then wipe the surface clean with a lint-free cloth. 

For very dirty surface: Pre-wash with soapy water. Use undiluted vinegar directly on hard-to-remove water spots. Apply the vinegar to a cloth and rub the area clean. Polish with a dry cloth.

Thanks to: http://greenliving.nationalgeographic.com/natural-homemade-streakfree-window-cleaners-2457.html