Disconnect the Vista drive and make the other (blank drive) master in bios AND jumpers.
Insert the XP cd and install.
After install...
change the drives back like they were and your off and running.
Not sure if you need to edit the boot.ini on vista or not, but in case vista doesnt see xp on the other drive, you need to use bootmgr.exe (i believe it is) in vista and add the xp drive to the boot ini file. OR dont do this and use your key that starts your boot order to boot from one or the other.
