Hi,
Does anyone know anything about replacing the ropes and pulleys in double hung windows with a spring system?

Hello Homeowner,
Yes, I do know how it is done and will tell you after you tell me why you are wanting to ruin a perfectly good window. Please don't take me as being sarcastic but I am a trying to keep you from losing the value of your house by replacing the cord and weight system.

Now for the directions; First remove the window stop on each side of the inside window. This will free up the window so you can tilt it in and remove the rope on each side from the recess and allow the weight to gently lower to the sill plate. Then remove the inside trim that is covering the opening for the weight. Remove the weight and untie the rope and pull it back through the pulley. Now there are two screws that are holding the pulley in the opening on both sides, remove them and the pulley, but do not throw them away. You or someone else might want to replace them at a later date.
Now there are two kinds of springs that you can use. One is shaped like a flattened out U and the two spring portions glide on the sash on each side of the window. The second spring is like a screen door spring and you connect the bottom of it in the hole where the knot of the rope was with a screw allowing the spring to travel up the groove that the old rope was in. Reinstall the window and replace the window stops leaving the springs sticking up past the window. Replace the outside trim over the weight hole. Now raise the window and prop it up where you wish it to be and then secure the springs on both sides of the window in the top holes of where the screw used to be for the pulley. Don't forget that when you release the latch the window will spring open without any help.
I do hope this has helped you with your situation.

Respectfully,

Ralph Jones