Moaning Murtle, N-Headless Nick, and the Bloody Baron, just to name a few, are some of the ghosts in HP now...
When Harry asked Nick about Sirus becoming a ghost, he said that it was a spell that those who fear death can use and stay as ghosts, now, how could Moaning Murtle a mere student at Hogwarts have known this spell, and have had the skill to preform it???