No matter how you cut it -- a good window is still a lousy wall. Glass has almost no thermal resistance (RSI - 0.16; R - 0.88), and a single-pane window in Canada not only makes a lousy wall in winter, but it makes a lousy window too -- you can't see through the thick layer of ice! But a house is not a home, without windows.
On the positive side, windows:
-- Let in natural light.
-- Let us see outdoors.
-- Provide free passive solar heating.
-- Act as a humidity alarm.
The major drawbacks of windows are:
-- Air leaks around the moveable frame.
-- Air leaks through the hole in the wall.
-- Conductive heat losses.
-- Occasional overheating of the house (for free too).
Our objective should be to maximize the positive aspects of windows while trying to eliminate or minimize their drawbacks. In an old house that was not planned with these priorities in mind we can encounter some problems.
If the windows you love are the wrong size or on the wrong side of the house, you may have to choose between good lighting and a panoramic view on the one hand, and energy conservation on the other. Windows on the north side lose to much heat; windows on the east and west lose in the winter and overheat in the summer; windows on the south side can heat in the winter and be shaded in the summer.