When you see windows that are foggy and difficult to look through, do you assume that they need a cleaning? This is not always the reason, often it can be due to a failed seal that keeps the gas between the panes of your double-pane windows inside. Double-pane windows are so efficient because of that gap between the panes, but if the seal around it fails, moisture and air make it in and cause that foggy look.
There is no easy fix for this problem. In fact, the only method for re-sealing the windows is quite expensive and far from effective. The windows will not get back to factory condition, so the only good solution would be to have the windows replaced, which is even more expensive.
The reason the seal fails is that the windows keep getting heated up and cooled down during the day which causes them to expand and contract, eventually breaking the silicone seal.