Most OEM (Original Equipment Manufacturers) provide their desktops or laptops with pre-installed version of Windows XP. You can simply plug-in and start using your genuine version of XP and you don’t even need to connect to Microsoft product key activation server to activate your serial number. OEMs attach a certificate of Authenticity (COA) to the systems using System Locked Pre-Installation (SLP).

OEM-SLP activation method gets done offline and end user does not need any internet connectivity to activate Windows XP on the installed system. However, the problem arises when one user decides to format the existing XP installation and make a fresh installation again using a CD that was not provided by the system manufacturer. In this case, the system looses all data regarding the activation and the installed Windows XP is no longer automatically activated. End-user need to use COA key to activate Windows XP installation again but OEM pre-activation might not work properly with original OEM recovery CD after the replacement of a defective motherboard.

However, Microsoft has prepared a guide for licensed end-users, which can be found here and can preserve OEM pre-activation when reinstalling XP. Follow the guide to know the steps to recover your old Windows XP product key and use it to activate your fresh installation.