First you need to take your cat to the vet to rule out an infection or any
other medical problem
Second Please don't post in (rich text) html  some newsreaders can not
handle it

Once medical problems are ruled out than the behavioral problems can be
dealt with.  Change in bathroom habits can be a major sign of a medical
problems

http://www.fanciers.com/cat-faqs/behavior.shtml

http://www.fanciers.com/cat-faqs/general-care.shtml

http://www.fanciers.com/cat-faqs/health-care.shtml

http://www.maxshouse.com/Database_toc.htm
this is Phil's site  a lot of information

above are links you need to update your self on cats

These are hints below and are not meant as a reason not to take your cat to
the vet which needs to be done.

Have you changed the brand of kitty litter you are using  if not try
changing the brand

You have to really clean the areas that the cat is uisng the detorant spray
does almost nothing urine and feces leave a stain that soaks thru carpet and
tile

Is there new stress in the house  such as  you moved furniture or a lot of
traffic in the household  etc   cats react to stress as humans do.

Put out two litters.  One with the same you always get and another with
a new type of litter(try the wheat type, all natural).  How much litter
do you put in the container?  Put at least 3 inches or try a new type
of box that she will want to use.  Try putting the normal litter in a
cardboard box with the top open.  Cat's like hidden places like behind
the entertainment center.  If she still goes behind the entertainment
center, put one of the litters there.

Put out two litters.  One with the same you always get and another with

a new type of litter(try the wheat type, all natural).  How much litter

do you put in the container?  Put at least 3 inches or try a new type
of box that she will want to use.

The first thing I would do is change the litter to a new type such as
the wheat one.  If that doesn't work try putting the litter in a
cardboard box with the top open and a hole in the front for her to get
in.  I've heard some cats don't like the clumping litter as it makes
them messy and is unhealthy when they lick themselves and ingest it.

It may just be a case of not liking what's in the litter you put for
her now.  There are chemicals and perfumes she may not like.  That's
why I recommend you try the unscented wheat litter.

Cat's like hidden places like behind the entertainment center.  The
walls of the cardboard box offers her some cover.  If she still goes
behind the entertainment center, put one of the litters there.

How often do you change the litter?  If it smells to you it will smell
even worse to her so it's time to change it.

Has your cat been declawed?  Some declawed cats don't like the "feel" of
litter. There should always be two litter boxes in different places.

                 ---MIKE---