When it turns into a creature, it is no longer an enchantment.
It changes into a creature even if the spell is countered.
It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
Standard: Ej legal
Pioneer: Ej legal
Modern: Ej legal
EDH / Commander: Restricted
Brawl: Ej legal
Historic: Ej legal
93/94: Ej legal