Making your XP version legit

I am trying to help a friend who is having some major computer issues. She was given a computer with XP home loaded. She has bought a copy of XP.. how can she make it legit?

 

if she bought a legal copy and wants to change the product ID in the current install call microsoft. or just reinstall windows with the new version she bought.

 

Thank you for your reply. If she reinstalls, will she lose all her data?

 

She can reinstall over the current copy of windows and keep the data but it wont be very stable it is best to format and reinstall which will lose all data so just back it up first.