With a vista/xp dual boot, Vista is the OS controlling the boot sector. While you may need to fix it after the format, because I suspect it is located on your XP partition, if it was installed correctly.
If you have a working Vista disc, it is as simple as formatting the partition, restart with Vista in the drive. From there open the command prompt, and use:
You could also try the automated version located on the Vista disc, but I have had little luck with it.
(more detailed instructions on using bootrec are located in the dual boot tutorial: http://www.syschat.com/dual-boot-vis...eady-1946.html
on page 3 of 4)