I'm having this problem with my 05 Mustang GT. It was parked in the driveway a few months ago. While I was at work, the sprinkler system decided to break and mist water on my car for nearly twelve hours before it was shut down. It was 105 degrees that day. The hard water is so bad, it had basically etched into the windows and paint.
I've tried a bunch of diffenert things on the windows: window cleaner/polish, simple green, bathroom cleaner, vinegar, 0000 steel wool.
For the paint, I also tried clay bar, which was supposed to remove swirl marks, but really just gave me a ton of swirl marks. I tried polishing compound with a buffer......nothing. I tried Meguires Fine-cut Cleaner with the buffer, which started to cut through, but didn't quite do it. Next attempt is either Medium-cut cleaner, or Meguires speed glaze. If that doesn't work, I guess I'll have to wet sand it.
The water in my neighborhood is nasty. The hard water build up is a pain in the butt in the shower too.