I would use a razor/utility knife to score and cut through the paint where the sash meets the window frame on all three sides. You may need to get a little more aggressive with a flexible serrated kitchen or steak knife. Once you have cut all the way through the paint then gently try to lift the window. If it is still stuck then try prying it up from the outside. Don't put too much upward pressure on the top piece of the bottom sash (from the inside) trying to force it because you might just disconnect it from the sides. As Jay said, be careful this is almost definitely lead paint so vacuum it up, have it tested and don't keep children in the house if you can help it. Hopefully all this will work. You will still be left with the question of what shape are the windows in? Some preservationists spend a lot of time and money restoring old, messed up windows while most folks just get replacements. There are many options for this. You can yank the whole frame and replace everything or just the sashes and tracks while insulating and calking/sealing the existing frames and weight boxes.