There is a special saw made for removing the paint that gets between the window frame and the stop molding. Any home center should have it. Once you get some space opened with the saw, you should be able to get a stiff putty knife and/or a pry bar into the gap to gently release the paint. When the paint seal has broken, the window should be able to slide.
Some old windows have pins that are spring loaded to go through the sliding frame and lock the window. You might check for these as the original pins are very evident but weird things can be jammed in to replace lost pins.