Make sure you are paying attention to your windows routinely. They can have issues going on that aren't obvious, so you have to pay close attention. If there are problems with the windows, then you want to have them fixed as quickly as possible. This can mean having someone repair them, or having them completely replaced. Here are reasons why windows may need to be repaired, or why they might need to be replaced: 

Why windows may need to be repaired

The glass gets foggy a lot - It can be normal for you to see a bit of fog on the glass every now and again. But, seeing fog often tells you that there are spaces where the air is coming in from outside. This is generally something that you can have repaired. 

The glass is damaged - If the glass has cracks, chips, or it's broken, then you want someone out. A broken window needs immediate fixing, or the window isn't going to keep the outside air from coming in. Also, any pests or even animals who want in your home can get right in. Cracked and chipped glass is now weakened, so the window should be repaired with the installation of new glass. 

There is a small amount of water coming in - If you find that any amount of water is getting in the house through the windows, then this is something that can and should be repaired. There are spaces letting air in that need to be sealed off. 

Why windows may need to be replaced

A large amount of water is coming in - If you find that quite a bit of water is coming into your home via the windows, then they will likely need to be replaced. A large amount of water indicates that the spaces may be too large to correctly fix with repairs. Water that gets inside can damage your home and your belongings, so this is something you want to get right on. 

There is damage to the window - When a home has wood windows, there is a chance of the wood windows eventually being water damaged. When the wood is rotting, the windows should be replaced. You may want to consider vinyl windows next time, as they won't rot in the future. 

Also, any type of window can end up damaged. It can be bent in a break-in attempt, it can be damaged in a serious storm, or it can be damaged due to years of use. When you see damage, you should call someone out to perform a new window installation for you.