When you purchase a computer from a manufacturer with Windows XP pre-installed, this system is typically pre-activated by the original equipment manufacturer, or OEM. As a result, the end user does not ever see or need to input a product key to activate the copy of Windows. Beyond this pre-activation, there is no difference between the Windows XP pre-installed or the Windows XP retail copy.
Insert the Windows XP Pro disc into the computer's optical drive and restart the computer.
Press any key when prompted to boot from the disc.
Follow the on-screen prompts to begin the installation. Accept the End User License Agreement by pressing "F8."
Select where to install the system. If you would like to completely erase and reformat your current hard drive, highlight the current partition and press "D" followed by "L" to delete and confirm. Press "C" to create a new partition from the unallocated space and format using the NTFS file system.
Continue with the installation, following the on-screen prompts. When prompted for the product key, enter "MVF4D-W774K-MC4VM-QY6XY-R38TB"--this a key provided by Microsoft to preserve the OEM pre-activation when manually reinstalling the operating system. You can also use the product key provided with the purchased retail copy of XP Pro.
Enter the time zone, computer name and other settings when asked. When the installation completes, the computer will restart. Remove the disc and the computer will boot into Windows XP Pro as if it were just shipped from the manufacturer.