Often door and window hardware has special brass or antique finishes on it. When you spill some latex paint on it and don't get it wiped up right away, or it has been painted over time and again, stripping off the paint can be touchy because if you are trying to restore it to its original condition, you don't want to either scratch it or use strong chemicals that might change the colour or shading pattern, particularly with antique style finishes.
The key is simply to go to the drug store and get Isopropyl Rubbing Alcohol. Soak the hardware overnight and rub off the paint with your fingers, or a non-abrasive dish cleaning pad. Use your fingernails, or tooth picks, for the cracks and corners. It comes off easily and without affecting the original colour of the metal finish.