workplace bully. what to do?

This woman is all Miss "sweetness and light" on the surface... when I first met her she was nice to me too. Then after a few years, out of the blue she is nasty to me when no one else is listening. She is a sadistic *****. She says the nastiest things, in a double bind sort of way and it hurts me deeply. I can not say anything to anyone, because no one would believe me. I have done different things to get her to stop, including stopping her and saying "did you just hear what you said to me?", and nothing works. Today I tried to give her something and she was nasty... Fine.. I have tired to not talk to her, but her desk is now right next to mine. And the cold air coming from hers is upsetting too. I swear, if I didn't know better, I would think the woman possessed. She is the most evil human being I have ever met... but all sweetness and light on the outside. What, short of strangling her when the boss is out having a cigarette, can I do??