You really need to be more clear on this question, I'm assuming you mean to upgrade from XP to vista but the way you worded it could just as easily mean installing Vista in a virtual environment under XP.
If you want to upgrade then rik is right, insert the Vista disk and follow the onscreen instructions. Be sure to choose the "upgrade" option because any other is likely to erase all your data.
No one has posted to this discussion for at least three months. Please let old threads die and do not reply to them unless you feel you have something new and valuable to contribute that absolutely must be added to make the discussion complete. Otherwise, please start a new thread in this forum instead.