When a Windows XP system wont boot up, you can often fix the problem using the Recovery Console. The Windows XP CD is bootable and will provide you with access to this tool.
To boot from the Windows XP CD, insert it into the CD-ROM drive on the problem system and reboot the computer. The system should present you with the option to boot from the CD, simply follow the prompts that will allow the loading of the basic files needed to run Setup. If not, you will need to access the BIOS and enable booting from your CD or DVD drive.
When you see the Welcome To Setup screen, shown below, press R to start the Recovery Console.