If my son has high functioning autism, is it likely my daughter will have it too?

Yes and no. Asperger's and autism may run in some families. But not in all families with one child with the disorder. If it runs in famlies it is likely to be due a multiple factor inheritance not a single gene. In short the only way to tell is to have your daughter tested by a psychologist with expertise in this area.
One one hand... ...It does run in families, and has a strong genetic component. On the other, the parenting skills you learned in the meantime are very important, and might make a huge difference.