When the gift of the Holy Ghost functions

"While the Holy Ghost can influence all mankind, the gift of the Holy Ghost functions when:
     • an individual has been baptized and confirmed by one having the 
        priesthood and has been given the gift of the Spirit,
     • an individual continues to act under the principles of righteousness,
     • the recipient is striving to be virtuous and pure,
     • the recipient acts with integrity and truth and keeps the commandments 
        of God."

- Elder Robert D. Hales, Of the Quorum of the Twelve Apostles, "Gifts of the Spirit," Ensign, Feb 2002, p. 12
http://www.lds.org/ensign/2002/02/gifts-of-the-spirit?lang=eng