Repairing old rope and pulley windows
(I got this from someone else's reply to a question like yours, which I read in this message board- somewhere in the second page of these messages)
the gist of the information at that link is that the pulley system is easy to repair. you can access the pulleys by taking the side trim off the window so you can see into the wall. You can replace broken sash ropes with new cotton rope or chain. To take the sash out, you remove the vertical sash stops that form the channel the sashes go up and down in.
If you want to replace the pulleys with something else, maybe Marvin Windows and Doors is what you are looking for. They have something called a Magnum Tilt Pac which apparently is a replacement sash that you can use in existing double hung window frames. http://www.marvin.com/