Now that dual-core processors are gaining in popularity, there has been some confusion over whether a dual-core processor counts as one or two. This discussion of multicore processor licensing may clear things up. The short answer is that a dual-core processor still counts as one processor.
For example, Windows XP Professional supports up to two processors. If you have two dual-core processors, Windows XP will use them both, for a total of four processing units. And if you enable hyperthreading on those processors, you get eight virtual processors out of the deal! Similarly, Windows XP Home supports one processor, but you if your one processor is a dual-core processor, then it will use both cores.
Now, I'm not so lucky as to actually have a dual-core machine, so I'm just taking it on faith that the linked article is correct. I haven't been able to run experiments to confirm.