"I am not interested."
I think this is because 感兴趣 is a verb meaning "to have interest in sth." while 兴趣 is the noun of "an interest".
So when you say: 我不感兴趣 --> "I'm not interested" (the act of not being interested)
While: 我没有兴趣 --> "I don't have an interest" (you don't have the noun of interest in something)
I hope that's not confusing. It's subtle.
Not much difference. But you can add more details after 没有兴趣 like “我没有兴趣(去)看书”. I guess this is due to the word “没有”.