OK, if I read you right your saying that evil prevails and muggles (non-magical people) are, by definition, evil in the Potter books? If so you really don't know anything about the books.
Muggles are portrayed as ignorant of the magical world around them (read that as "real world" within the context of the stories) and can sometimes be frightened or pig-headed about things because of that. They are most clearly not all evil. Harry's aunt and uncle are typical, money grubbing, materialists, but they are not inherently evil. In fact the family that represents "good" wizards, the Weasleys, is headed by a man whose life is devoted to understanding and protecting muggles. He's a little daft, but clearly a good guy.
The force of evil is reserved to Lord Valdemort and is represented by the Malfoy family of nasty wizards. They are the ones that preach the hateful doctrine of wizard purity and not mixing with muggles, etc. So far the forces of evil are 0 for 4, so they aren't exactly winning. They're 0 for 5 if you count Harry's initial defeat of Valdemort as a child.
Now, I understand that people are easily decieved and that the occult is a tempting path that has ensnared many unwary people. However, in the context of this thread, I have to agree that the Harry Potter stories have far more in common with CS Lewis' overtly Christian literature than it does with anything promoting the forces of darkness. Belief in miracles presupposes a willingness to accept supernatural forces exist. Setting a fictional story in a world where those things happen doesn't immediatly mean that the author is promoting satanism. Context and intent are important, though obviously sometimes difficult to discern and open to interpretation.
If you've never read C.S. Lewis' works I'd recommend, for adults who enjoy speculative fiction (also referred to as Sci Fi), starting with Out Of The Silent Planet, the first of his space trilogy. Imagine the War Of The Worlds written by a devoted Christian. He has also written some excellent non-fiction books which are very worth reading and thinking about.
If I misread your post... never mind.