When Harry was playing nice and clean with his new firebolt, Malfoy and his company tried to disturb Harry by scaring him. Malfoy, who dressed up as a dementor, tried to scare Harry and knock him off from his broom.

With these circumstances, Harry managed to perform the Patronus Charm, a mist of silver cloud enlarged and hit Malfoy.

Was Malfoy affected with the spell? But a patronus charm is specialized for dementors.