No, Terry is actually right. While "many times" the boot files may be on the XP drive, they're always on the Active Partition.
Generally speaking, the active partition is the C:\ drive of the first partition you installed. In your case, with XP first, Vista second; the active partition is the XP one. But for anyone installing Vista first and XP second, the boot files will never be on the XP partition (unless they use HnS).
Generally speaking, the active partition is the C:\ drive of the first partition you installed. In your case, with XP first, Vista second; the active partition is the XP one. But for anyone installing Vista first and XP second, the boot files will never be on the XP partition (unless they use HnS).