after running the knife down the edge.
next hold the blade at a 30-45 degree angle and run it down the paint. this cuts away a section and creates space. again, try to avoid cutting the wood. you may not need to do this stage if the windows are not too badly painted up or if the gap is narrow. if the gap is wide and/or very full of paint this is a good way to get it out.
the second image shows the window after this stage has been completed.
you may also need to cut around the top; this can be rather tricky. get hold of the sash and gently pull 