While shutting down, the computer saves some system files, however, due to some reason these system files may not be stored properly. So, the next time when you start Windows the error message shows up..
could try this first..
Boot from the windows XP CD, press the "R" key in the setup in order to start the restoration console. Select your windows XP installation from the list, and enter the administrator password.
Enter the command: "FIXMBR" (without the quotes) at the input prompt and confirm the next question with a "Y" (without the quotes). Use exit to restore the computer.