it was definitely reported before the car was due a service

I bought a car almost 9 months ago from a Volkswagen dealer and as it was second hand i got a years warranty with it. Since i purchased it, i have been back to the dealer 4 times with problems with my windows. On two occasions the window has stopped working completely. Other times the window goes up and then for some reason comes down again by itself (electric windows). This can be very distracting as I drive in a town a bit and put the window down a bit when inside the 30 mph zone, but when i head out onto the motorway i put the window up as join the road, but sometimes the window shoots down and is very distracting. I have been told 4 times that the problem has been fixed by the dealer. The last time they were told to keep the car until they could guarantee that everything worked. When on the phone i asked the service rep if he could guarantee me that the motors werent faulty, that the wiring wasnt faulty and that the switches werent faulty. He told me everything was working perfectly. I took the car home on friday and on saturday the window was faulty again. They have also been told that if this problem happened again i would be looking a new car as the one they had sold me was faulty from the start. They have been asking me to prove that the car was serviced by a VAT registered company and before it travelled 10,000 miles since i bought it. As this has happened again im fed up with it and im wondering if anyone can give me any advice as to my legal standing in this issue and what options are available to me. Thank you.