First off, just want to say that all credit for the knowledge of what works goes to WhiteIrocZ. He told me this information, I just chose to use it and document it for a write-up.
What you need:
A 15 OZ. bottle of Weiman Glass Cook Top Cleaner ($4.30 at your local Wal-mart) Can be found with the other cleaners in the grocery side.
A pack of ultra ultra fine steel wool. This is grade #0000, it is very important for you to get the right one. The pack I bought had 12 pads in it and is made by Rhodes American. You can find this also at your local walmart for $3.00. It will be with the sanding stuff in the paint aisle.
A cloth. This can be microfiber, terry, or a plain rag from the bathroom. It doesn't matter as long as it is clean.
Some elbow grease. You will have to really bear down in some spots, depending on how long your windows have been spotted up.
Here are pics of the supplies you need to pick up:
Cook Top Cleaner:
Steel wool:
Not great pics, but just remember to pay attention and get ultra fine(0000).
Process:
First, you need to at least wipe down your windows, not really getting them super clean, but so you won't scratch if there pretty dirty.
Second, take the Cook Top Cleaner and pour it on the window in little dots a few inches apart. Don't put it everywhere, but you don't have to spare it, and one bottle sill be plenty. It is almost like waxing, do a little at a time, maybe a square foot of window.
Third, scrub with the steel wool, it will not scratch up your window, but your hard water spots will vanish.
Last, take your cloth and rub up all the excess Cleaner with it. If you still see spots, put more Cleaner down in the spotted areas and scrub a little harder.
It took me about 45 minutes to do the back hatch glass, about three on each mirror, and about 20 on each side window. I have not done the windshield yet, it got dark on me.
Now for more pics:
I only have one before, I didn't think about doing a writeup. But, you guys all know what water spotted windows look like and how much they ruin an otherwise beautiful car.
After:
Hope you enjoy my write-up. This stuff really works on hard water spots.
Well I for one gave it a try.
Definitely a good write-up, but I was still a little skeptical.
Out of curiosity, I tried the cook top cleaner by itself and it did nothing. Then I tried the steel wool by itself and it doesn't do anything, but when you use both together as the write up states they work magic.
My car was a DD for many years, always sat outside, and had some very bad waterspotting. I said had...here's a pic with the in progress result!
im going to go and get that stuff tomorrow.
i have tryed the zaino glass polish with my pc with a 4in yellow pad and it shines it up but it dont get the spots out.
it seems to work so its worth a try thats for the info
wouldnt your windows be all scratched with the wool pad ?
#0000 steel wool doesn't scratch the glass, just make sure the glass is clean, as the write-up states and don't try it on your paint/plastic, they are too soft!
This definitely does get out the water spots entirely, but it takes about the same amount of work as compounding by hand.
Yes, this will work great and will not scratch. It is pretty easy if you take your time and work with it a little. It is fun to see the spots vanishing.
I have some bad water spots on my windows but I am not sure if it is on the inside where the tint is or if it is on the outside.. I have rolled my windows down when its been wet so I didnt know if it was on the inside.. This will scratch the hell out of my tint so I hope its on the outside lol.. Maybe I will try this sometime..
Hard water spot removal write-up. 
mods: Torquer 2 112lsa, ls7 Clutch, 3.73's , underdrive and altenator pulley, Ls6 intake, pacesetter longtubes and offroad y, magnaflow exhaust, qtp electric cutout, lid, ported and polished throttle body, and free mods...
