Advantage has always worked totally and very fast with all of
my cats no matter how many fleas they had.

-cr

Frontline works wonders. I remember fighting fleas before Frontline was
available, this way is sooo much better. I think it is only available from
vets. I've heard Advantage is good too.

why not?

You need a multipronged approach.  
    1) Get a hightech antiflea medicine like topspot.  I've found flea collars
    to be worthless;  ditto for flea powder.  Don't use a hartz products;
    they're deadly!
    2) vacuum every square inch of carpeting, throw the bag out
    3) use a flea comb while waiting for the anti-flea medicine to break
    the flea's lifecycle.
    4) wash every single scrap of clothing.  Don't leave any dirty clothing or
    sheets unwashed.
    5) vacuum the entire house again, throw the second bag out too.
    6) if you live in an infested area and have an indoor/outdoor cat, comb
    kitty everytime you let kitty in.