mate, are your drives IDE (wide ribbon cable?)
If so, on the back between the power and ide plug sockets is a little row of pins (usually eight) with (probably) a little white jumper on them, printed on the drive will be a picture of the jumper settings showing which pins to jump to achieve master/slave etc.
If the drives are SATA (red cable) there is no need to worry about master/slave, just ensure your XP drive is set to first boot device in the boot order in the bios.
If you have trouble with jumpers I suggest you don't touch your boot.ini file, the easiest way for you would be to reinstall Vista, it will auto configure your BCD (boot.ini) when you install.
From within XP, drop your Vista disk in the drive, let it auto run, select the destination drive (real important you get the right drive or it will wipe out XP) and away you go.