Consider replacing your windows and doors only if the damages are beyond repair. Window kits and other weatherstripping products will help you reduce heat loss by as much as 50%. Take a close look at the weatherstripping and mobile parts of your windows and replace them if they are used or damaged.
Windows & Patio Doors

We can easily attribute 25% of all heat loss to poor quality windows. Consider these three easy solutions:

•  Seal vertical slider windows with « V » or tubular shaped weatherstripping. Also weatherstrip around the frame.  

•  Transparent film that shrinks with the heat of a hairdryer considerably increases the window's air tightness and also increases its thermal resistance. The film helps prevent frost from forming and leaves the window nice and clear.

•  If you don't use your patio door in the winter, cover it with a sheet of shrink film. Removable sealing products are also available for patio doors.