I'm seen at work here with an impact screw driver. A tool that has proven it's worth several times over. I'm also using a high torque cordless drill and an impact wrench but the impact screw driver always comes out on top for loosening the really stubborn ones. These screws are actually so stuck due to galvanic corrosion caused by the use of steel screws in aluminum window frames and water leaking past the old window gaskets acting as a conductor. To lessen these effect all screws are replaced with stainless steel ones and are all coated with Loctite Anti Seize to make removal in the future easier.

Photo courtesy of Jon Snyder