I know there are other ways to fix this problem, but here is a complete solution that you might want to consider.
01. Insert the Windows XP CD-ROM.
02. Restart your computer.
03. When prompted to Press any key to boot from CD, press the Space bar.
04. At the Welcome to Setup message, press R to start the Recovery Console.
05. If you multi-boot this computer, select the installation of Windows XP that failed to start.
06. When prompted, type the administrator password and press Enter.
07. When the command prompt opens, type bootcfg /list and press Enter to display the entries in the boot.ini file.
08. Type bootcfg /rebuild and press Enter to scan your hard drives for Windows NT-based installations. Follow the on-screen instructions to add installations to the boot.ini file. Example:
Total Identified Windows Installs: 1
[1] C:\Windows
Add installation to boot list? (Yes/No/All)
At the (Yes/No/All) prompt, respond with Y.
09. When a you receive a message similar to Enter Load Identifier, type the boot menu description and press Enter.
10. When prompted to Enter OS Load options, type /fastdetect and press Enter.
11. Type Exit to restart your computer normally.