The problem is pretty common with all cars, unfortunately. It's caused by two things. The rubber strips that the window sits in in the window frame are generally lined with a fine, fuzzy felt and the windows can easily slide against it. Over time that fuzz wears off and the windows are rubbing against stickier rubber. The other problem is the metal rails the window rides on inside the door get dirty over time and can slow windows down as well. The fix is to clean the tracks and rubber as best you can and lube them up to lower rolling resistance

You can check out
this thread (which links to another) for more info and feedback.
