This is the simplest way to make piratet Windows XP Genuine. The rar archive contains two files: 

WGA.reg <-- Double click, and allow it to make entry in your regestry.
Microsoft Genuine Advantage Diagnostic Tool.exe <-- Use this to verify that your Windows XP has become genuine.

Currently, I\'ve just tried this on the Pro SP2 edition, but seems to work on all versions.

Take care!