Enter the Windows XP product key near the end of the setup process, after entering your name and organization. Type the 25-character product key into five boxes, with five characters per box, without dashes. The product key is normally written on the original Windows XP retail packaging.
Entering the XP product key during installation grants you a 30-day license to use the system, after which you must register, or activate, your copy online.
You may also update the XP product key present in an existing install using the Windows Product Key Update Tool. This is useful for fixing activation issues or if you entered a blacklisted product key during the initial install.