You can start by checking for loose items to see if anything's binding.
If the window's half down, can you rock the pane fore and aft? If so, examine the track mechanisms. There are some replaceable plastic guides at the base of the scissor to check. There are also some mounting and adjusting bolts to make sure are tight.
Otherwise, try checking the archives. Jeff's right - Bill's posted some regulator replacement instructions. I think you'll want to hope that's not your problem.
Bellevue WA
90 535iM - not much stock remains. 3.7 liters, ported head, cammed, 3.73 diffy, M5 brakes, MAFed, yadda yadda yadda
86 Porsche 951 - Track Toy