The PC Magazine site has an article on setting up a dual-boot system. It concludes with two warnings:

Dual-Booting Win XP and Vista

Finally, Two Warnings
1 The BCD provides a more robust start-up environment for Windows, but the downside is that if you want a multiboot system, you must be sure to install Vista last. If you put Vista on a machine and install Windows XP afterward, Vista won't start, because Boot.ini, which Win XP adds automatically, will render the BCD useless (although you can recover it by running the fixntfs.exe utility from within Win XP).

2 Even when your Win XP/Vista machine is happily dual-booting, one problem remains. As you work in Vista, the System Restore utility automatically creates restore points—and you can create them yourself—that let you launch the system in a previous state and recover from errors. But when you exit Vista and launch Win XP, the older OS deletes the Vista restore points.