Hero Quest - Adaptation to remove 'Evil Wizard'
Boy do I love Hero Quest -- that's the game that started it all for me.

I've tried playing Hero Quest without Zargon a couple of times, and it's tricky but doable.  

You basically need one person playing both a hero and Zargon, and trying to move the monsters as impartially as possible.  For example, if you enter a room with two Fimir, and its their turn to act, don't have them run off afraid, or have them move up and then not attack.  Either the hero/Zargon, or the entire group, need to come up with what you think the monsters would do.  

You can either come up with a strict attack order (monsters will go for the Wizard first, Elf second for example) or dice off to see which way the monster will go.  

You might have to re-write or re-copy the scenarios with the treasure notes and secret doors and traps written down elsewhere, so you can't cheat and say "Well this treasure chest is trapped because I read it right here" or "Hey, there's a secret door here, I just read about it".  Alternately you can put the traps and secret doors on the board, but you need to roll some target value to even be aware that it's there.

There's a site called BowserBag.com that has some interesting links, including a PDF for play without Zargon, found here.