Multi-core processor and multiprocessor limit for Windows XP

Hi Luigi A. Cruz,

The number of processors you can use depends on the version of Windows XP you use, though there are some caveats. Be sure to know what you're doing before you attempt to set up multiple processors.

Dual-Core

Microsoft's licensing policy limits the number of processors Windows supports for its Home and Professional versions, as outlined below. It's important to understand, however, that this is on a per-processor basis, not a per-core basis. This means that, under the licensing policy, a dual- or even quad-core processor counts as a single processor---something that confused many people in the early days of dual-core technology.

Versions of Windows

Knowing that, the limit of the number of processors is determined by your version of Windows XP. If you're not sure which version you're running, check the sticker on your computer. It will say the version there beside your license key; you may right-click "My Computer" and click "Properties." The window that pops up will tell you which version of XP you're running.

XP Home

The Home edition of Windows XP---the edition that came with most machines home users bought from the likes of Dell and HP, before Windows Vista---supports only one processor. This means if you want to install more than one processor and you run Windows XP Home, you may need to switch to Professional, or upgrade Windows to a newer version with support for multiple processors.

XP Professional

If you want to use two processors, Windows XP Professional is the way to go. While this advanced program cannot run more than two processors, this is an improvement over Home. Note that, because dual-core processors count as a single processor, you could theoretically have four cores with this license or even eight, if you are willing to buy two quad-core processors.

I suggest you to visit this below provided Microsoft KB article.

Processor and memory capabilities of Windows XP Professional x64 Edition and of the x64-based versions of Windows Server 2003
http://support.microsoft.com/kb/888732
   
Also refer:

XP Home supports 1 processor and XP Pro supports two processors, regardless of the number of cores.
http://www.microsoft.com/licensing/about-licensing/multicore-processor-licensing.aspx#E1B

They can also check the max number of CPU’s on their own PC by going to Start > Run, typing  WINVER, and pressing the Enter key.  Then click on "End-User License Agreement".
http://social.microsoft.com/Forums/en-US/genuinebenefits/thread/1b6ea167-e9f1-4551-8793-0c86f1e6eed5