James and Lilly a ghost?

[meckelburg at foni.net]

No: HPFGUIDX 15352

You've probably discussed this at length before, but I only joined 
this group yesterday and I desperatl want to know:
Why do some people change into ghosts when they die and others don't?
We know Myrtle and Nick died by force, but Prof.Binns just fell asleep 
in his Armchair. Myrtle and Binns died in Hogwarts, but I don't think 
Nick was executed there, so why has he, and some of the other 
House-Ghosts come to hogwarts after their death?
I hope I am not just bringing up really old stories and am anxiously 
waiting for answers

Mecki