I have watched a mate do it, but having seen him do it i would say it's fairly simply if slightly fiddly.
The hardest bit is removing the door card, you have to be very careful not to break the clips that hold the bottom bit on.
Once the door card is off, you just need to unbolt the window from the lifter, then undo the bolts holding the regulator to the door and remove the regulator and motor from the door. The motor is just screwed to the regulator and is just as simple as taking one off and screwing the new one on.
What is the problem with you window? I ask because of the three mk3 golfs i've had experience of all of them have had the same regulator problem. The part that connects the bit of the lifter bolted to the window to the regulator is made from plastic with metal inserts, the plastic become brittle and breaks separating the window from the regulator. The symptoms are the window doesen't work or work sometimes, it might struggle, or the finger trap cuts in for no reason. Although you can hear the motor running.
If it is your motor it's not a hard job, probably a couple of hours in all.
