well, there is no magic for these windows as these are mechanical devices...

checkpoints: (now, if there has been any glue poured down the sills, well there is the answer,, otherwise:

1) Window guide rails.. lack of lubrication (however, it's a 2002 car?? so, I doubt that...still possible though!)

2) Power - (a whole can of worms)

a) liquid spills? b) metal pieces or anything dropped i.e metal dust?

b) Fuses and connectors.... In this case, check for loose connections / loose fit.

Finally, the weather / noise gaurds on the doors where they meet the glass... sometimes, if harsh chemicals have been used to clean windows, they may deteriorate the rubber and cause it become elastic creating a friction for glass from gliding easily.

3) Worst Case:  Door frame bent or Guide Rails bent... this will happen only during an accident or someone purposely "twisting" the door...

Not much else I can think of.