If your motherboard's BIOS *has* a boot selection option, often you can select F8 or F11 or something during boot to bring up a menu to let you pick which drive to boot from. Also you can often configure the drive boot order / priority in the BIOS menus themselves. Consult your motherboard's manual for details.
Turning OFF the BIOS logo display so you can actually SEE the options helps.
Even if you install them on different drives if you've installed XP first, Vista may give you troubles installing if it doesn't think you're installing it on the first "boot" drive of the system. This really isn't master / slave / primary / secondary, it is just whatever one the BIOS is configured to define as the "first" boot drive in the system. Vista may refuse to install on anything else. You can unplug (temporarily during vista install) the other drive or switch the drive boot priority in the BIOS if they bios supports that to let Vista install on the correct drive.
Vista, if it sees an XP drive in the system at the time you're installing Vista, MAY try to overwrite the XP drive's boot record to go to Vista's boot selector code. I'm not sure if it will or not in your case. To be totally safe just unplug the XP disc during Vista installation. On the other hand if it did replace it and it works as intended it'll pop up a menu at boot asking which OS to start, Vista by default or XP otherwise. YMMV.
Otherwise if you install XP after vista you'll just have to use the BIOS to switch which one to boot unless you install a boot manager to let you switch..
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