I had exactly the same problem. Start the engine and let it idle then try and do the window can you hear the engine labor? if so sounds like the motor is stuck.
What you need to do, there are 4 allen key screws on the motor housing, mark the side of the housing with a pen or somthing then remove the four screws / bolts on the housing then remove it.
Now get a can of WD40 and give the motor a generous spraying. Start spinning the motor by hand now, this should feel quite stiff, do it a few times each way until you are happy.
Now put the housing back on noting the mark you made and the side it went (most likely facing you) and put the bolts back in and test it before you put the door card back on.
Ashley Sandell
April 2008