It isnt recommended to install 2 OS's on the same HDD. What will happen is that the files for XP will become .old files and the Vista will be the ones used. Which in turn can corrupt your XP isntall since it is trying to use the current Windows files which are actually for Vista.
You are better off getting partition magic and making a seperate partition for this. It will save you a LOT of hassle in the long run.