"Interested at" also sounds wrong to me. I can only think of it being used when "interested" and "at" happen to be used one after the other. For example: "I'm not interested at all." "I know you are not interested now, but you might be interested at the end when you've heard more about it."

Could you give us the sentence with "interested at"? It sounds like the wrong collocation.

If somebody is interested in/at something, it means he wants to know more about it.