Take some baking soda and vinegar and make a paste out of it.
Then apply the baking soda and vinegar paste to your windows or any other glass that has hard water stains.
Then use a scrub brush or towel to scrub the window or other glass until the water stains are removed.
Then take some vinegar and water in a spray bottle and spray the vinegar and water on the glass to remove the rest of the baking soda paste and to clean the residue off the window or glass.
For really stubborn water stains you may have to repeat this procedure.