99.9% of the time
Vista will automatically repells the boot of the xp
it's also a good idea to install on 2 different hard-disk
let say you have a 100GB named C
and another 100Gb named D (2 indpedent hard-disk)
then you could install XP on C and Vista on D
later change the setting so that you could choose your boot-system when you boot your pc