Parking under a tree may give you some shade, but it can also cause quite a mess on your windshield and windows. If you can't get that sap off with a rag and some water, try nail polish remover instead.
Image: Kaushik Narasimhan.
Nail polish remover is great for all sorts of things (after all, it's a pretty strong chemical), so if you don't want to go all the way to the auto shop just to clean up your windshield, try taking some acetone nail polish to it first. Don't try this on your car's body, though, as the acetone can take the paint off your finish. In that case, you may have to suck it up and get some bug and tar remover, but if it's just a case of some messy glass, nail polish remover will do the trick in a pinch.