If the open windows are of the tilt and turn type then the best advice is to leave them because IF their window leaks they'll try to claim off you for damaged carpets, wallpaper, etc, etc. It's not worth the hassle if they can't be bothered to remember to close their windows if they know you are coming. The customer won't thank-you personally for cleaning their open window (through thoughtless ignorance on their part), BUT they will be on the phone to you and will have left countless messages on your answer machine if you bugger up their carpet or laminate floor,etc. My saying is: 'If it's open, sod them'!