I am in my third year of my PhD and I have had enough of my supervisor. She is possibly one of the most unprofessional and horrible people I have every met. She treats me like her personal assistant/ slave. She makes me run errands for her whch have nothing to do with my PhD or the lab for that matter. If I am unable to help her I get rude comments from her nd she makes me feel very guilty even if I have very good reasons not to be able to help. I'm fed up of being treated like this and need help to take this further. I'm scared of making an official complaint because I am sure she will ruin me and stop me from completing my PhD. But what else can I do?

I think it wouldn't be a good option to complain about your supervisor. Even if there is a chance for that in some universities, it would surely reflect badly on this academic relationship with the supervisor, especially that as a supervisor she has a final judgement on your thesis. I think what might be practical is to avoid clashes with her as much as possible and to act wisely. Maybe you can try to focus on developing your thesis and apologize for tasks that you cannot fulfill.

I agree with skyhoo, for how tough it is you will have to deal with this person for some time. Don't forget that supervisors are usually also referees when you apply for jobs after completion.
I would suggest to :
1) Keep contacts to a minimum and to be always professional and polite - tough but possible
2) Don't bad mouth her - I know it's very tempting thing to do, but you usually pay the price for doing it
3) Focus on completing your thesis in the best way you possibly can
4) Build a strong network around you - it will be valuable support on many levels.
Situations like yours are unfair, but sadly very common and not only in Academia. For how tempting it is to drop everything or to confront her you need to develop skills that enable you to go ahead without damaging yourself and your future career.

Rather than create further angst for yourself and get yourself into a psychological state that incapacitates your ability to complete your research, why don't you change supervisors. I suggest doing a general search of available supervisors and meet with those who have similar research interest as your own. Prepare for this meeting by developing a set of discreet questions that will give you an indication of their working style. I would also suggest that you discreetly share that the current relationship is no longer meeting your research needs - be very diplomatic! Don't get into the situation where you do bad mouth your existing supervisor. Also,ask other candidates about their recommendations. Once you make a good match and the prospective supervisor agrees to supervise you, then follow due process to change supervisors. This way you will ensure you have the level of support you need to get you through the rest of your research and you are not wasting energy, time etc being consumed by an untenable situation.