In tip 10768 » When you run the Windows Genuine Advantage validation check, you receive 'Windows Activation Required'? AND tip 6623 » How do I change a Windows XP pirated key to my legal corporate or retail key?, I described some reasons why WGA (Windows Genuine Advantage) does not validate your copy of Windows XP.
Microsoft Knowledge Base Article 916247 contains the following description:
The Windows Genuine Advantage (WGA) validation check process does not validate successfully on a Microsoft Windows XP-based computer. A resolution is provided.