Pull off the trim on the interior and exterior of the window using a crowbar or the claws of a hammer. The trim is nailed on so slide the claws behind each nail and pull it out. Remove the trim gently if you plan on reusing it.
Take the window glass out of the window if possible. This varies, depending on the type of window you are removing. Slide the window so it is open halfway so you can grasp the pane on either side. Push the window up and pull it away from the frame. Newer windows have locks at the top that you have to slide before you can move the window.
Unscrew or pull out the nails that are holding the window frame in position. Use the claws of a hammer or a crowbar to pull out the nails. Use a screwdriver to pull out the screws. If you can't get the screws or nails out, use a Sawzall to cut through the nails between the wall and the frame. This frees the frame.
Pull the window frame out of the window with the help of an assistant. Use a wedge and crowbar when necessary to help shimmy it out of the wall.