Had the same problem with my windows. What you need is Maplin part N71AN - Silicone spray with PTFE. You can buy the stuff on the Maplin website. Once you havee it:
Wind the window down
use the straw to spray it into the gap in the rubber strips around the edge
of the window openning.
You'll find it costs a fortune if you get it from Maplins (if it's all you're buying) cos the postage etc added on. I got some from motorspares, but MAKE SURE ITS SILICON, as that's best for lubricating agains rubber.
It helped loads (had to spray quite a bit in though) but about 6 months down the road it now needs doing again.
Thanks for reminding me!
Slow Side Windows - Also how to replace the electric motor mechanism.
2 options
Firstly is Toms way (and easiest)
Maplin part N71AN - Silicone spray with PTFE
Wind the window down
use the straw to spray it into the gap in the rubber strips around the edge
of the window openning.
Second option
This is easier with the mechanism removed
1. Remove the door panel (see above)
2. Take off the inner lining
3. Unclip the window to guide puller clip - its in the middle of the window and is a bit of a fiddle - it is a sort of plastic loop, you may need to use some pliers.
4. Unplug the wiring loom to the motor.
5. Undo the three mechanism retaining bolts then wiggle it about and extract it through the apperture. You can leave the window in place.
6. Clean off the centre runner with some degreaser and then regrease using a light grease.
7. You can check the mechanism off the car by simply plugging it back in. You may also need to regrease the worm behind the cover.
8. Put it back into the door and bolt it back up.
9. Reclip the window to guide.
10. Spray some silicon based lubricant into the side runners as well.
Next time I do one I will take pictures as it is a bit awkward.