If there are significant areas of damage, it can be costly and time-consuming to repair windows. Often, in these cases, it can be a better investment to replace the windows altogether with something more robust and energy-efficient.
Some common problems with your existing timber windows might include them being warped or draughty, and broken seals may well have led to condensation between the panes. They may also be coated in layers of paint which could potentially be affecting how well they function.