This is actually a really good question... loaded too. I like what LT and Kitten said. I believe there are different types of evil spirits. In Heaven, we know there are several types of angels... and these are by no means the only types among what seems to be billions of angels. Just based on the titles the Bible uses, we know that there are at least princes (because of the reference to the existence of the chief prince over Israel: Michael), chief princes, archangels, cherubim, seraphim, and guardian angels. On the other side, Paul at least tells us that there are classes of evil spirits: principalities, powers, etc. I believe, from some experiences as well as a bit of practical sense, that there are different types of evil spirits. I think the different translations have stumbled in trying to distinguish them with terms like 'demons, unclean spirit, spirit of an unclean demon', etc.
Vic, I think that since most of the Bible is 'in part', you can safely view many things that are written as being incomplete (incomplete means not everything was explained and spelled out). That passage does say that the angels are kept in chains, etc., but then the Bible also talks about territorial spirits and even princes (in the OT) as well as 'gods' and thrones and dominions. It's safe to say that a demon can't successfully oppose an angel; therefore, the 'prince of Persia' who opposed successfully a messenger angel (thought to be Gabriel) cannot have been less than a former angel (i.e. a fallen angel) himself. Apparently, not all angels are bound in chains but a certain number of them. Consider that if all angels were bound, then satan would have to be bound too. It seems that only certain angels were allowed to continue here on earth while the rest (I'd say a specific group or the greater percentage) was bound up in chains in darkness till the day of judgment. I think classes should be offered on these kinds of topics; but not many people understand these areas, and there's not much teaching about things like this.