It's likely that the flea eggs were left in the house by previous
inhabitants with pets. I have heard that the eggs can last for a
couple of years undisturbed. Frequent vacuuming will help to hatch
the existing flea eggs.

I believe the company "flea busters" uses boric acid in it's home flea
treatments. You can use the same methodology at home by sprinkling
the powder onto carpeted areas, then beat it into the fibers with a
broom. Frequent, regular vacuuming will help in alleviating the
problem.