As stated above, if you really want to polish your windshield, you need a cerium oxyde powder. I've used this kit before with great results.
http://www.caswellplating.com/buffs/glass.htm
There are a few considerations:
- If you're talented with a rotary and you have a good assortment of polishing compounds you can do a good job. You need to build up some heat in to polish the pits in the glass, the objective is to make the edges of scratches and pits smooth so that they don't catch light as much.
- Do not EVER try to polish side or rear windows. The single pane tempered glass will shatter if localized heat is applied to it. The process only works on windshields because they are two coats of regular glass laminated with a film in between.
- Go ahead and wax / zaino / glaze / whatever your windshield with a PC or orbital, it won't harm it and the mild polishing action will clear up some swirls and such. It's going to help making it water repellent.