The Rebuild command checks an installation for valid startup information and files and can help bring an XP installation back to life.

There are two ways to recover from an unbootable/invalid installation; a simple way that works most but not all of the time. Then there is a more complicated but more thorough method.

1. Boot from your XP Setup CD and enter the Recovery Console
   (a slipstreamed disk will work as well also)
2. Run "Attrib -H -R -S" on the C:\Boot.ini file
3. Delete the C:\Boot.ini file
4. Run "Bootcfg /Rebuild"
5. Run "Fixboot"

base on: "XP's Little-known 'rebuild' command" by Fred Langa, InformationWeek April 17, 2006