Cris,
You can try some "Bon Ami" household cleaner or a specialized windshield scrub at your local autostore. Most of them are apply, agitate, then hose off. Once you have done that you may also want to go over the windshield with something like STP Vision Blade. In my experience it really helps at night in the rain or on the highway. Swapping in a new wiper blade or insert at this time should make the windshield the best it can be.

Hope this helps...Lee