Harry's Parents Were Not Killed By Voldemort

[Tuima]

I don't think the manner of death has very much to do with becoming ghosts. The impression I got from Nick in OotP was that those who were afraid of "the next great adventure," as DD calls it, chose to stay behind. If you had had an unpleasant life or something you really wanted to finish (i.e. Myrtle was "determined to haunt Olive Hornby"), you might also stay. But Harry's parents were brave people who had lived well and died doing the right thing. I think that they would chose the Next Great Adventure, and not become ghosts.