Last year, after we adopted a dog from the rescue, we also acquired a large flock of fleas. We were concerend about using poisons as we have a 3 yo roaming the carpet, so we did some research and found a product called Fleabusters. I believe it is nothing more than borax, but it did the job, we've been flea free ever since.

If you want to try it with good ol' 20 Mule Team, the instructions were to sprinkle liberally around the carpet (especially under furniture) work it in with a broom and don't vacuum for 3 days. Supposedly the little buggers cut themselves on the sharp edges of the granules, and then are dessicated by it.

Good luck.