Well, that depends! If the window screens are metal, then you can use a standard paint stripper like Zip Strip. However, if the screens are the newer fiberglass type, the paint stripper could very well damage them. You might want to try just the same. If the stripper damages them, you'd just have to replace the painted screens, which you might have ended up doing anyway.
In either case, use the stripper carefully when removing paint from window screens. Be sure to work in a well ventilated area, wear safety glasses and follow all label directions.