Quoting Craigular.B
Just wanted to do a quick update-
I did a quick Google and a lot of the guides I found around the web put XP on /dev/sda4 instead of 3 (they use the diskutil command interface to make 3 partitions), and install XP on the last partition instead of the 3rd. They still use the same dd command, though. Would that maybe be it?
You could try...perhaps XP requires installation to the last partition (I vaguely remember this to be true), where Vista is more tolerant. The tutorial is for Vista x64, so you may have to adapt it to XP.