Removing A Double-Hung Window

Double-hung windows only look complicated. Most of them have counter weights concealed inside the frame. I have not tried to describe windows that have springs instead of weights or those that have a complex type of zinc weather stripping installed in the 1920's. The basic structure of all double-hung windows is the same.

Slide a thin cats paw or putty knife behind one of the front stops. Gently lever it away from the frame. Long pieces of wood are fairly flexible, but if you work in small increments up and down the entire length of the piece you are less likely to break it. Remove any protruding nails from the frame. You only need to remove the stop on one side.
Secure the chains or ropes on each side of the sash by inserting a nail through it to stop it from sliding back through the pulley. You can fix broken ropes later.
Lift the sash up and pull it towards you. Unfasten the rope or chain from the edge of the sash
Removing the upper sash on a double-hung window can be frustrating. Crack the paint seal between the sash and the frame and remove the parting bead which is either: a) nailed in b) painted in c) a very tight fit or d) all three. Work on it with a putty knife and pliers that have their jaws wrapped in tape to protect the finish. Most lumberyards sell replacement parting beads. If yours breaks, take the pieces to the lumberyard and get a new one cut to length. In an ideal world, the parting bead fits snugly into its groove (which you can see once it's out) and is held in by friction alone. When you do get it loose, wiggle it past the bottom of the top sash, pull it out and set it aside.
Slide the upper sash down past the pulleys and secure the chains or ropes as in step two.
Lower the sash and pull it forward. Remove the chains or ropes from the sides of the sash as in step three.
Once both sashes are out of the window frame, pry open the secret doors in each side of the frame. These are the weight pockets. I am sorry to say that to open both pockets you might have to remove both parting beads. Once they are open you can replace your dirty, broken ropes or rusted, painted chains with flat sash chain that lasts forever. There should be four weights in all - one for the lower and one for the upper sash on each side. New weights are hard to come by - see if your old weight is somewhere inside the pocket. Most hardware stores sell sash chains and they can tell you how to attach them. Many will also be able to tell you where to find used sash weights When your window was first made, each sash opened smoothly and stayed put. The pulleys were fastened to the frame and well oiled. The chains were attached to the sash at one end and to the weight at the other. They ran smooth and flat over the pulleys. The weights and chains were not tangled up inside the pockets. The chains were long enough to let the sashes travel up and down and short enough to keep tension on the weights even when the window was closed. With only occasional maintenance, this low-tech mechanism will last for another hundred years.
Now you understand how a double-hung window works you can re-install it by reversing the steps. When everything is back in place and running smoothly, make sure that the lock that holds the sashes together in the middle is tight and in the right place. Locking the sashes provides security and keeps out the cold.