If that doesn't work, go into the Command Line Recovery Console and run the following commands.
bootcfg /rebuild ---- Rebuilds Boot.ini
This scans all system hard drives for installations of Windows and allows you to add them to the boot.ini file.
If the XP install is the primary partition, you can run the "fixboot" to reinstall the boot loader of the partition.
You can also use the "fixmbr" if the XP partition is the primary booting partition.
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