An older post but I'll still throw in my $0.02.
I learned this as I used to do professional window cleaning...as long as it'* only your windows you're using it on, standard blue toilet bowl cleaner (i.e. Sno-Bol) works wonders for removing hard water spots from glass, use a soft-scrubbing sponge and use the "wax on-wax off" method. After that'* all done, all it takes is regular window cleaner to remove the toilet cleaner and get those things shining! (this is also handy if you have this problem with your home'* windows)
PS: Don't get it anywhere but glass! A little will go a long way!