You need to go back and learn to read more carefully  JUST IN CASE HERE IT
IS AGAIN

"First of all that last stupid dumb a.s comment about putting her down  take
it and throw it out the window.  If you honestly feel that way please take
your cat to a no kill shelter  and give that cat a chance"
Ok now that has been said  take and separate the cat till the cat calms down
by could take a couple days  put water dish food dish and a litter box for
the cat into a bathroom or separate room  no one goes in for at least 24
hours
Is this cat fixed?
When did you adopt her was this recent?
If this was recent  how did you introduce the cat?
Are you willing to call a animal behaviorist to come in and help?
This may take a while to readjust the cat to your home?

Now lets try this again

Your cat has been nasty for 5 years?
 Since you did not say this is the same behavior for last 5 years.
Something has caused the cat to get worse  the other cat  how long has it
been there.
Is it possible for other family members to have caused a problem with the
cat specially young children or is there a unusual amount of stress in the
house

In article
<4e6a4082026330e24b9b7b972374b6db@localhost.talkaboutpets.com>,

I urge you to get the cat to a vet for a complete workup and
toxicological screening. If her behavior changes were sudden, it could
be a sign of an internal physical event (such as poisoning or stroke or
other problem), in which case euthanasia might be a humane consideration.

Pets like yours can sometimes act as the "canary in the coal mine" for
homeowners, heralding an immediate threat to the human family. You won't
know unless you have a vet examine her. At the very least, you can thus
eliminate possible physical causes of her behavior.

As far as the cost, find a vet who is familiar with the various
diagnoses of aggressive feline behavior, so that they don't waste a lot
of time on extraneous tests and examinations.

The latest issue of "Catnip" magazine carries an article on aggressive
behavior in multi-cat families and lists some of the causes and cures
for it. It should be available at your local library.

Here is an excellent book on correcting aggression and other kinds of
behaviour problems in cats:

http://www.amazon.com/gp/product/0553378546/104-9496789-9901558?v=glance&n=283155

http://search.barnesandnoble.com/booksearch/isbninquiry.asp?z=y&pwb=1&ean=978055
3378542

After you have the thorough check-up at vet to rule out medical conditions,
talk to him/her about the ideas suggested in this book.  As per the book, as
a last resort, some animals end up having to be put on medication such as
valium, etc. for awhile.

And here is another one written by Pam Johnson-Bennet, a feline behaviour
consultant:

http://www.amazon.com/gp/product/0895947102/104-9496789-9901558?v=glance&n=283155

FIRST  PLEASE NEVER GIVE ANIMALS HUMAN MEDICATION UNLESS DIRECT SO BY A
QUALIFIED VET

ONLY IF YOU HAVE A VET Administer is it and the cat is under constant vet
care .  My cat rumble is on it  for epilepsy and it can causes liver and
kidney damage test must be given every three months to make sure both are
functioning correctly.  Valium and diazepam are both the same just in case
for reference it is very hard on a cat  systems and can very well send a cat
into a stroke or worse.  Mine has to take it due to he is allergic to the
other common prescribed for epilepsy

Phenobarbital is more commonly given instead to cat  but not always for your
cat  problems  there are other medications that a vet will prescribe  after
a full work up is done   there are also natural products that an animal can
take.  I can almost guarantee that the vet will basically prescribe a downer
drug so to say.  The vet will know best.  Depending on what area you are in
there are low cost clinic available  but we need to know what general area
you are in to show links in you don't mind telling us

In the last 6 months have anything change in the house such as new members
moving in or out, moving around furniture  moving  familiar items to another
spot. Moving the cat box or food dishes to a different location. Cats are
sensitive to environment change  and some don't respond to it well

A couple things to try to help the cat calm down  move its food dish to  a
spot away from the other cats if it is close maybe a separate cat box in a
quieter location for her.

The cat attacking the other cat is extremely common when the other cat has a
different smell on them.  Our cats give each other wide areas when they come
back  what we do it get some worn clothes and wrap the cat in it and give
them a human scent bath.  Makes the other cats less aggressive and makes the
cat give itself a bath and get the new smells off of it.

That is something to discuss with your vet.  I think the dosage would be very
small for behavioural problems but it may cause liver problems in some cats.
In the book by Pam Johnson-Bennett, she suggests Bach flower remedies (a
health food store product) but never having used them myself, I can't tell
you if they work or not but it may be something you want to look into also.  

Here is some info (a vet's column) regarding aggression and drugs used to
treat it:

http://www.vetinfo.com/catcataggr.html

By the way, I have a cat that is friendly over all but will bite if I pet him
too much, cut his nails or brush him.  He had to be on valium for a few days
for a medical condition a few years back.  Although the dose may have been
higher than what would be used for an aggression problem, I didn't like him
on the drug as he seemed "dopey" and not himself.  Also, I recall a cat that
was put on it at a shelter I worked at for behavioural problems and she was
not kept on it very long as she became clumsy (fell off a table) and the
staff there missed her old "spunky" self.