It may become necessary to update the XP serial/license number. If Windows XP is being used, then changing the XP serial number is easy. You can learn to change the XP serial/license number in a few easy steps. In Windows, the important details about the operating system and applications are stored in the registry. Some knowledge of Regedit is needed, to learn to change the XP serial/license number. Make the required changes in the registry and run a command to change the serial number. Alternatively, tools or utilities such as Windows XP serial number modifier can be downloaded for free to make changes to the XP serial/license number.
Open the Start menu and click Run
Type regedit and click OK
Note: Before undertaking any operation affecting the registry, it is recommended to make a backup of it
In the Registry Editor, click File > Export and save the backup somewhere you can easily find it