The filly I just bought is VISIOUS! HELP!

I agree with what everyone has said. The first thing you need to do is gain her trust.

When we got our warmblood he was a nasty piece of work and didnt want human attention at all. We spent time sitting with him while he ate, brushing him while he eats, talking him for walks and doing as much as we could to show him all we were going to do is fuss over him and love him.

Spent a lot of time with her. As kelly said tie her up to brush, pick feet etc. if she isnt into bring brushed everywhere just get a little further each day. Even if it takes you a month to be able to brush her belly then so be it. To you she might be vicious but to her she is protecting herself from possible harm. You have to work hard to be the person that she knows for sure is not going to hurt her.

Don't rush anything. Go at her pace. You have all the time in the world. While she feels comfortable keep going but as soon as she isnt comfortable anymore stop and do something she does like.

Always remember that to her you are a predator until you prove yourself as a friend.

Oh yeah and I wouldnt recommend round yard work at this point in her life. Maybe using the round yard as a secure area to do things with her is ok but I wouldnt be doing a lot of circle work in there unless you have a 40ft round yard

Ok, call me nuts if you want to but I believe horses
Learn from you interacting with other horses.

Like get a well mannered horse and go through the routines of
Picking up hoofs and cleaning etc etc etc. and have the horse
You are wanting to teach close so they can watch you and the
Other horse. They learn by seeing the other horse stay cool and collected
So it turns into OK, this person will not hurt me type of lesson.

To me the first thing is to remember the horse is a prey animal
And the fight or flight is always first in their minds. (self presversation.)
You have to over come this instinct they have, so there is when the word Trust comes into play, They have to trust you and you have to trust them too. That leads to bonding with each other.

All this takes time and patients so do not rush this.

In a full size round pen, you don't have to worry as much about baby joint issues, especially if you are using techniques to gain trust and respect - rather than just running to burn extra energy. I agree the round pen is a good tool if you know how to use it. If you don't, I agree that it might be best to bring in a professional (what is a little upsetting now, will be incredibly dangerous later).

A little beasty eh Ah first off, do not give up and don't let frustration get to you. You will fix this she just needs time. You are rehabing that poor thing and she simply doesn't any better. As far as she is concerned, every time you get near her, it's do the same thing that little fart did to her.
I would work her in a round pen. You don't need to get near her, she doesn't like it and she doesn't trust you so you need to work on that trust and for safety purposes, distance is key. You can do a lot with horse in the initial stages without touching her.
Work on in a pen, get a lunge whip and get her working on the whoa and moving forward when you say so. If she decides she doesn't want to stop, then do the opposite and keep her moving forward and you decide when she stops. The key is YOU decide what she does. Work with her in short sessions as she is a foal and always leave it on a positive note. It shouldn't take her long for her to catch on and realise it's so much easier to just go with the flow but might take more time for her to realise you aren't going to beat her.