If the window were a pivot style window, and balanced in an open state, a pump and a pair of ballast tanks might be enough to open and close the window. A pair of slender chambers (Melinda suggests containing oil) could be inserted in to each of the vertical pillars of the window. Even better the pair could be the top rail and the bottom rail with tubes running between in the verticals. Figure 1 shows a piston tank which uses a rotor motor rather than a pump.
Fig. 1
Alternatively some sort of mechanical counterweight might be good. Perhaps a threaded lead counterweight on a threaded rod.