Personally, I believe in ghosts. I know that it's appointed unto man once to die and then the judgment. But what happens between death and judgment is largely a mystery to most.
I believe that witches really can conjure the dead. God's Law states....
Duet. 18:10-12
10There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch.
11Or a charmer, or a consulter with familiar spirits, or a wizard, or a necromancer.
12For all that do these things are an abomination unto the LORD: and because of these abominations the LORD thy God doth drive them out from before thee.
If there wasn't anything to it I'd think God wouldn't take it so terribly seriously, instead his law would reveal that it's not true. But the way the law addresses the subject, it would appear to be true that withes indeed conjure the dead.
Notice what happens with the witch of Endor...
1 Samuel 28:13
And the king said unto her, Be not afraid: for what sawest thou? And the woman said unto Saul, I saw gods ascending out of the earth.
When the witch conjured Samuel you'll notice other spirits were ascending out of the earth. This demonstrates that conjuring the dead opens a door way and that often additional spirits break free to roam on the earthly plane.