You can set your boot options in your bios directory. Your talking about installing into a seperate partition and not another HD, I'm assuming. When you install Vista it will take priority over XP when you boot up. Vista will automatically become C:\ drive and XP will then be labeled D:\ or some other letter. It doesn't matter if its on another partitian or HD. The Vista bootmanager is not the same as XP boot.ini
With the Vista betas, RC1, RTM at least on my Asus it didn't pick up my XP after install so I had to manually set this up using VistaBootPro
http://www.vistabootpro.org/ and set up your XP legacy OS. If you install Vista and decide you want to uninstall it or only boot into XP, you will have to go into Recovery from your XP CD and fixboot. If you do this you will no longer be able to boot into Vista. Aconis Disk Director Suite has an operating system selector that also works with this process and Acronis also allows you to delete partitians very easily.