I would let things settle a little, don't panic just yet. I've had a few friends go through similar things and usually the girl calms down after a few days and becomes more reasonable. I can guarantee you that threatening and involving lawyers will tick her off and make things much worse. In the end she's probably got just as much right to ownership as you do, even if you paid she could say it was a gift which will match the registration.
Relax and let her calm down, she'll come to her senses and eventually do the right thing, I would bet on it. I'm sure she was once a nice person, she will be again, girls just get crazy sometimes, eventually you'll get use to this
