Problems with the master boot record (MBR) of a system may prevent the system from booting.
The MBR may be affected by malicious code, become corrupted by disk errors, or be overwritten by other boot loaders when experimenting with multiple operating systems on a host. This tool describes one method of repairing the MBR for an XP host using the recovery console.

Boot with the XP installation CD.
When prompted, press R to repair a Windows XP installation.
If repairing a host with multiple operating systems, select the appropriate one (XP) from the menu. If you have only one operating system, enter 1 to select it. Enter the administrator password if prompted. To fix the MBR, use the following command:
fixmbr

This assumes that your installation is on the C:\ drive. . Type y and ENTER to fix the MBR. Type exit to leave the recovery console and reboot.