It is probably because you have installed one of the service packs. (SP1 or SP2). This would be normal.
If you have backed up your data, and you have the XP serial number, then the best route would be to format and reload.
Make sure the XP disk is in your CD Drive, make sure the bios is set to boot from CD, then restart the PC, and keep pressing the space bar to start the installation process. Follow the instructions and select the option to format. After that the installation will commmence.