If you have a brick veneer house (one layer of brick with framed walls on the inside) the windows should be attached with a nail flange. A sawzall and some blades for metal will do the job.
Remove the sash, cut the head jamb as far as you can and then pry it downwards (yopu may need to work the saw in and cut a few nails). Cut through the frame completely, then start cutting nails and pulling the frame into the opening.