I have used a boric powder (basically borax) in the carpeted areas. Fleas eat it, they can't digest it, and it kills them.
But before you even go that route, you need to treat the cat with Advantage or Frontline. Do NOT use an over the counter product, but get something from your vet. Over the counter products can cause more harm than good to your cat.
Second, wash all bedding, throw rugs, and anything loose in the house.
Third, vacuum, vacuum vacuum, and don't forget the cracks and crevices when you do this. An exterminator once told me that this is the absolutely best way to get rid of fleas. Throw the vacuum cleaner bag out when you are finished.
I used boric powder for a few years and found I didn't really have to do this at all if I did the above. It helped control them, but using the tips above eliminated them.