Official Microsoft statement on http://www.microsoft.com/licensing/h...multicore.mspx
Q. What is a "processor"?
A. A physical processor is a single chip that houses a collection of one or more cores. A core is a collection of one or more processor threads and a set of shared execution resources. A processor thread is the architectural state within a processor that tracks execution of a software program thread/task.
Q. How does this licensing policy affect products such as Microsoft Windows XP Professional?
A. Microsoft Windows XP Professional and Microsoft Windows XP Home are not affected by this policy as they are licensed per installation and not per processor. Windows XP Professional can support up to two processors regardless of the number of cores on the processor. Microsoft Windows XP Home supports one processor.