If you can detect a definite draft coming from around your window that means that moisture can get through as well. This can cause water damage and facilitate the growth of mold, which are both expensive problems to resolve.
In a dual-pane window, if water droplets or water vapor collect between the panes, it means that the seal which bonds the panes together has failed. This failure means that the window will no longer be energy efficient. The gas that was trapped between the panes (argon) is now gone, replaced by air immediately cutting energy efficiency by 30-40%.
Touch the window. If it’s always warm to the touch in the summer and always quite cold in the winter, it means that the window is not insulating your home properly. This puts stress on your heating and cooling systems, increasing your energy costs.
Look for cracked window frames (especially at the corners) and look for cracked glass - either of these issues will cause your energy bills to rise noticably if they affect multiple windows in the home.
When inexpensive vinyl or fiberglass single-hung, double-hung, or sliding windows no longer slide open or closed with ease, they are typically not salvageable. They are now permanently warped by the settling of the building and the repeated expansion and contraction brought on by the changing of the seasons and corresponding changes in air temperature and direct sun exposure.
Replacement windows not only improve the appearance and value of your home, but they can also help you save considerably on month-to-month energy bills, and prevent mold growth and water damage within your home. Moreover, your utility company will provide rebates and perhaps other benefits as a way of encouraging you to make your home more efficient by replacing the windows.