My two adopted kittens had fleas when I got them. Summer was horrible.
What worked was going to the co-op and looking for an insecticide called "Permethrin". Its sold under a variety of brand names, and is toxic to fish and all insects. Plus, it may or may not be a carcinogen in humans.
But it works. Dilute it according to the instructions, vacuum your carpet first, and then spray every inch of floor in your house, as well as the baseboards. Open the windows and go for a walk in fresh air.
Vacuum every day, and repeat the spraying every 7-14 days. Spray the cats every 3 weeks or so. Flea comb the cats often.
There are only two non-toxic methods that I've had any success with. The first is a flea combing the kittens daily (dropping the fleas in a cup of soapy after). The second is using a nightlight and putting a pan of soapy water underneath it. They won't cure a flea problem, in my experience, but they will kill some fleas.
This winter, I only have a few fleas, so I'll probably switch to Frontline, which is very effective and should kill the few remaining fleas. Then I'll be flea-free until my cats escape outside and pick up a new batch.
PS: Everything that Walmart sells for flea control is a joke. Even the flea collars.