When is It Time for a Window Replacement?
Drafts getting in through shut windows is quite normal. However, such drafts aren't something that you can ignore for a long time. If the breeze entering through your windows become more intense over time, your window frame could be old or damaged enough to warrant replacement. The more you ignore this type of nuisance, the more your comfort would be compromised.
If you're having great difficulty in opening your windows, it could be a sure sign that you need a new one. The only place where creaking, stuck windows belong are in abandoned houses that have nobody to tend to them. Rust and mechanical failure are primary causes of hard-to-open windows, and should you choose to ignore this, you risk damaging the other components of your windows as well.
If you have double or triple glazed window panes, moisture or condensation in between the panels are rarely a good thing. Condensation indicates a seal failure, thus defeating the purpose of such windows in the first place: energy-efficiency. In this case, it's wise to just replace the window.