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:
This assumes that your installation is on the C:\ drive. You will be presented with several scary warning lines the reading of which will make you want to say no. Microsoft is exceptionally vague regarding the conditions under which fixmbr can cause problems although they are clear about the consequences (losing all data on the hard drive), so use this at your own risk.
Type y and ENTER to fix the MBR.
Type exit to leave the recovery console and reboot.