are you able to boot into Vista?
If you are then the boot files are on the vista partition, ( this is normal as the default is to place the boot files on the start-up of the newest OS ), and it should be no problem to proceed with the re-install of XP as described in the step by step.
If not and you have the original Vista disk, use this to perform a start-up repair on the Vista
install. when you have this completed, and vista is booting correctly you can can re-install XP.
Some more reading for you I'm afraid
Startup Repair - Vista Forums
If you do not have the original Vista disk to do the repair have a look at the options here ...
Create a Recovery Disc - Vista Forums
Hope this helps