I wouldn't purchase the dewormer from Hartz but may try other brands. I would go to a pet store exclusively and look at their products. Hartz products have never been the strongest or the safest out there. If you live near a "co-op" they may have dewormer there to give. Co-ops typically have the same thing as vets do since farmers use these medicines routinely on their farms.
I believe Ring worm has to be treated by a vet but other common worms can be treated over the counter.