You need to think carefully before setting Vista up as a dual boot. It doesn't modify the Boot.ini file in the root of your XP partition but installs a number of additional files including a Boot manager. This screenshot shows my XP partition "after" and "before" installing Vista as a dual boot click here
Prior to installing Vista, I created backup images of my XP partition with both Norton Ghost and Acronis True Image to ensure I can get back to where I was.
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I ran the Vista Setup and installed it into "Unallocated" space on the drive, i.e. unpartitioned and unformmated, so that Vista could do the necessary partitioning, formatting and installing.