Dai's right. Bleach turns white stuff yellow if you're not carefull. I used to use it to whiten white shirts (in my army days) and you had to use a diluted bleach/water solution; but after many 'bleaches' the shirts would 'yellow' anyway.
Mind you, I've read here that window cleaners have used a toilet cleaning limescale remover to assist in cleaning hard-water marks from windows.
I tried it once, and although it didn't make the window look like new, it definately did remove some of the hard-water stains.
But for general window cleaning trad style, I don't think you can beat a good squirt of the good old Green Stuff.