I have actually installed XP after Vista (triple booting machine... Linux, vista, XP).
Heres what happens:
Windows XP install kills the Vista boot loader. You can NOT boot Vista from the XP boot loader. Even if you use grub as WanderingKnight suggested, grub can only chainload to the Vista/XP boot loader. Once you install XP after Vista, if you chainload to Vista, it'll boot XP and only XP.
On the Vista DVD there is a BOOT folder. Within that folder, there is a command line app called bootfix. Run that after installing XP to restore the Vista boot loader. The vista boot loader can boot XP but you'll need to manually configure it. It does not use the boot.ini of old.
You can use this to make it easier:
http://www.vistabootpro.org/