I need advice on handling harassment

I work at a pretty big international firm, but I've only been here for just over 3 months now. I know/knew one of the people who works here before I started. I've known her for 10 years. I was aware that she had some serious mental problems which she won't acknowledge thereby making it impossible to help her.

She is very good at soliciting sympathy from others and will even lie to make her situation sound worse in order to get that sympathy. For example, she would keep telling everyone she had cancer when they only removed a mole from her. Her child is always sick and she always works more than double the hours she needs to etc. In short, she is not the most uplifting person to have in your life.

Recently I heard about a scenario in which she physically attacked her husband in public - it was quite bad and he ended up with a bloody lip and blue eye. We did suspect it since there had been signs over the years but I never confronted her.

Now I think she may have found out I know about it, cause she has launched a huge campaign against me. She harasses me via email and phone, she has never had the self assurance to take someone on face to face or even by phone. She uses facebook and all other media to threaten, insult and degrade.

I've been ignoring her totally for a week now in the hope that she will stop, but she just keeps going. She attacks me and everyone close to me and tries to turn us against each other by spreading lies. When we don't react she gets her mother who is in the police to call us and threaten us.

I have no idea how to handle this! Also being at work with her is making everything harder for me. I have a lot of anger and maybe even hate towards her and I know that hurts me more than her since she seems to be enjoying this. She laughs when she sees me cause she knows I'm powerless against her.

Please please please, any advice or suggestions?

Sorry, I should've added that I suffer from depression, it just makes situations like this impossible.

Hi
I am so sorry you are going through this! I am not sure how the legal situation is in your country. I had a bad experience in the US and I learned a bit about it. Keep records of everything. Print emails and store them. Keep a log with all what she does to you, write down sentences, words,how she made you feel, date, place and who were with you at the time of harassment; start going to a psychiatrist or a psychologist to document how this is affecting your psyche, get legal advice, read about work harassment and know your rights, when you are informed about your rights and legally advised, then talk to your supervisor.
To me the process was awful and when I got out of it, I started with a depression I am still fighting. I know I had a condition before, after that the following jobs put a lot of stress on me, etc, but definitely this event made things worse, it was too stressful. Try to get help, the sooner, the better.
I wish you the best