I used
EasyBCD to set up my dual boot Vista and XP.
I installed each OS on it's own HDD, I then plugged them both onto my motherboard (plugging my Vista HDD into SATA connector 0 and XP HDD into connector 1 so that my machine boots up from my Vista HDD) and edited in XP onto Vista's boot menu using EasyBCD.
*You will also need XP files
boot.ini,
ntdetect and
ntldr that are found in your XP's C: folder (you will have to use Tools/Folder Options/View and add a tick in "Show hidden files and folders" and maybe also untick "Hide protected operating system files" to be able to view those files), just copy them into Vista's C: folder and you will be ready to edit XP onto Vista's boot menu and boot in successfully thereafter!