1. Boot computer from bootable Windows XP CD
2. when the first screen stop and prompt options, press R to repair an existing installation.
3. In the console type commands as below:
C:\> FIXBOOT C:
C:\> FIXMBR
C:\> BOOTCFG /rebuild
Enter Load Identifier: Windows Server 2003, Enterprise
Enter OS Load Options: /fastdetect
4. reboot done!
Related
http://support.microsoft.com/kb/317521/en-us?fr=1