If you use that program that fathomzero recommended you should be able to DELETE the vista partition and combine it with your xp one. So you could transfer all the needed files from the vista partition to your xp partition, and then just format the vista one and use that program to combine them.
also if you do that and vista transfers over you might be able to remove it through START/RUN/MSCONFIG/BOOT, but i dont know for sure
in fact if you dont mind having two partitions you might just be able to delete it that way :O :O :O