They showed me a product from Windowmaster Products, called EZ Frame.
In a nutshell, you remove the old wondows from your frame, but leave the aluminum frames (rails). The new window slides in this reduced opening, and is screwed through the aluminum frame into the studs.
Vinyl or wood trim when finishes up the inside.
Basically, a 4'x4' double-pane sound-insulated, sliging window with grids is window is about $250. A little expensive, but almost no labor to install.
A comparable non-retro window was about $150, but the labor or time would easily exceed a couple of hundred dollars.
My outside stucco would have to be smoothed down to accomodate the fit, but it sounds TOO good to be true.
Does anyone have any experience with this retrofit idea?