There are some things to try, including it seems an automated dual boot repair tool knocking around.
However, there is a very high risk something could go wrong and you'll end up not being able to boot into XP as well.
Back up everything and have XP CD and W7 DVD on hand, perhaps, in case everything has to be setup again (or Repair Install made).
Let someone else do it (preferably the persson who set it up) unless you have no choice.
Do you have another computer in case of problems?