Have I ruined my macbook screen by using windex to clean it?

Well, Windex certainly isn't good for the screen. You're not supposed to use any cleaning product on a laptop screen. However, the newer Macbooks have better screens. Also, the screen tends to dim over time.

Check the ingredients of your Windex solution. If it has ammonia, it has permanently destroyed the surface of your screen. If it's ammonia-free Windex, you are fine and it's probably just streaks. Use a moist lint-free towel to clean the streaks.

No you shouldn't use Windex. There is a special cleaner you can buy to clean it or you can just use water on a cloth. Don't worry so much about the brightness of the screen on your friends computer. Sometimes it's just like you said, she may have a better graphics card plus the fact that it's newer may have a lot to do with it. Sometimes materials change in the manufacturing process. The newer the materials, the better the quality of the picture. The fact that you do spray the spray on the cloth first is helpful though. If you didn't notice a drastic change in your screen then you probably didn't damage it. You would notice small abrasions or possibly end up with a blurred screen if you had damaged it. I suggest looking into the other cleaner although it is more expensive. Walmart carries it. Or just the damp cloth.

No you didn't ruin your screen, not they way you are doing it, because this is the proper way to wash a screen. However it is suggested using a soft lint free cloth and mild Dishwashing soap to clean the screen followed by a damp lint free cloth to remove the soap. You should no use a chemical product containing ammonia because it will streak the screen. The way Laptop monitors are assembled you are not actually washing glass, but a plastic sheet. There are five different sheets of plastic that sandwich the plasma glass screen.

"Never use windex to clean your laptop screen" try microfiber cloth instead

yes; windex has a substance that isnt good for liquidised screens.

It may be part of the reason, but only part. Since your machine was manufactured, there have been substantial improvements in image quality, including color and brightness. A big complaint about older Macbooks - and their PC counterparts - is how their screens "wash out" in bright light. There have been improvements in that, too. As long as your machine is usable, don' t toss it - use it until you really, really need a new machine. Oh - and knock off the windex bit - in fact, R.T.F.M., which will tell you to use nothing but a soft, lint-free cloth on the screen or one of those industry-approved cleaning pads you can buy by the boxful at your nearby Office Max store...