not sure without a picture, if you have the real glass windshield or a plexi-glass. if it's real glass, you could possibly use one of those flitz balls on a drill to buff it out.It could be some type of air intrusion into the glass, hard to say without seeing it.
if it's the plexi windshield- that's called "crazing" I believe. it's just a matter of time (usually a looong time after being exposed to UV) that causes it. and I don't think there is anything that can be done besides replacement.