If it's caused by mineral deposits, most acid-based cleaners (such as CLR) works well, so long as you don't drip it onto your paint.
If it's caused by some other things (e.g. industrial fallout, silicone deposits from synthetic wax/armerall), you may have to get something to strip it out. There are some car windshield wax strippers available and I personally know a couple of them but unfortunately, one of them can only be available from the Orient (by Soft-99).
Q.