1. Click the 'Start' button and choose 'Run' from the start menu.
2. Type 'regedit' in the text box, then press the 'Enter' key on your keyboard.

3. Find and click on the following registry key in the left pane of the registry edit tool window:HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\Current Version\WPAEvents
4. Right-click on 'OOBETimer' in the right pane of the window and choose 'Modify'.
5. Change any number in the value to deactivate your Windows XP operating system.
6. Close the registry editor, click the 'Start' button and choose 'Run'.
7. Type '%systemroot%\system32\oobe\msoobe.exe /a' into the text box and press the 'Enter' key on your keyboard.
8. Select the 'Yes, I want to telephone a customer service representative to activate Windows' option and click the 'Next' button.
9. Click the 'Change Product Key' option.
10. Type your Windows XP product key in the text box and click 'Update'.
11. Click 'OK' to complete the changes.
12. Open an Internet browser and go to the Genuine Microsoft Software webpage.
13. Click the 'Validate Windows' button and allow the website to validate your copy of Windows XP. If you have used a genuine product key, your copy should be validated and you will not receive Windows XP piracy errors.