Spot-X is one product that removed hard water stains from shower doors. Stains so bad the remodel guy said we'd need to buy new glass...the doors looked like they were factory opaque. After I cleaned the doors and put them on the porch to dry, he walked right past them without seeing them, they were so clear!
http://www.glass-restorer.com/

My auto glass guy sells another product that works well, slightly easier to apply and cheaper by half, A-MAZ water stain remover:
Pacific Sun Makers Inc. 1-800-921-6861
You might even make a few bucks if the water is so hard in your area, become a distributor!