If you're considering replacement windows, but don't have the budget for the whole house, don't completely close out the idea.  While replacement windows are a great way to improve your home's appearance and energy efficiency, but they can be costly.  If your budget is limited, begin by replacing windows on the side of your house that takes the worst seasonal impact.   Easterners start with northeastern windows Southerners, replace southeast windows first and so on.   You can replace the rest as your budget allows.