You need not assign the movie clip to a variable.._root itself means the main stage or the root..you are attaching your movie clip to the root or the main stage..so just _root.attachMovie ("aliens","aliens", 100); would be enough..also to remove or destroy the movie clip just specify its name..here it is aliens..
so aliens.removeMovieClip();
and for assigning x,y coordinates try it as aliens._x=random (180)+10;
aliens._y = -20;
I think you are working with actionscript 2.0..
I hope this works..
but I am not sure about it..