Here is a technique to change XP password of your friend's login pc without knowing any hint or old passwords:
just open CMD PROMPT and type as shown below
C:/ net user username newpassword <----|
for example...
I F UR FRIEND USER NAME IS " Cena" ....u want to change itz password to "Batista"
TYPE AS SHOWN BELOW
C:/ net user Cena Batista <====
tats alll u have successfully changed ur friendz password to "Batista"
If you are the admin of pc then go to CMD prompt and type
"""""control userpasswords2""""""
here you can change the password