To be honest, I can't remember whether it worked or not. When I began building my PC a few months ago I was confronted with this issue. I remember inserting the Vista DVD and running Startup Repair, but the EIDE hard drive where XP was installed went bad and had to be sent for repairs. Then I couldn't get XP to install on a SATA drive and that took a few days to solve. Between those and other issues, I lost track on whether or not the Startup repair worked, though dual-booting is working fine.
Just in case, I'm including a link from Microsoft, which details the step to take:
http://support.microsoft.com/kb/919529.