XP treats dual core chips like dual CPU systems, and aside from a few little details, they essentially are. Besides, it would be very difficult for Intel and AMD to market multi-core chips if the dominate operating system didn't support them. They've both been working with Microsoft for some time regarding this, and if need be, they would have created special drivers to make them work.
Now, whether or not the support for dual core chips in Windows is as good as you could get from an operating system like Linux is a little unlikely, but the simple fact remains that without Windows supporting it, it would be a very tough prospect to move these chips.