"How are you doing?" -is a very ordinary question. I have to hear it at least few dozen times a day, atleast six hundred to thousand times a month.
Answer is not so simple as it seems. Sometimes you might get extraordinary answers.
The answer depends on too many factors. How is your mood that time, your relationship with the person who asked, how old are you, how old is the person who asked, etc, etc. Actually it depends on how you are doing in fact, but a very little.

Tell me the truth, do you really know how are you doing? Lets say, God forbid, I will have a stroke tomorrow, what am I going to say today? Doing good? I might feel good but my BP and LDL/HDL level probably dont feel that good. Or lets say you have been yelled at, by your boss and you are thinking you are having a bad day. But in fact probably today, you are on top of your health. You sprinted to catch your train, you made love to your wife for significantly more time. So how are you doing good or bad?
"How are you", you get easy, simple and direct answer from kids. Although any question has straight forward answer for them, only you have to find out the truth behind it. For example, as a kid when I use to come home from school after a math quiz,
mom used to ask, "How did it go?", she would get a quick reply, "Good". May be I was able to understand I would hardly make it fifty out of hundred, but telling truth would keep me grounded for a week, so "Very good" and delayed the misfortune.
But for grown ups these answers can be very twisted. Paul is your friend. You meet Paul every morning on the 7:05am train to hoboken, "How are you doing", "Not Bad" you say. Now you relocated to hoboken and after 5 years you met Paul in a light rail. He asked "How are you doing?", you want to say "Worried about my high BP, and 30 lbs weight gain. My daughter is going gothic. My little lab passed away etc". Probability is you will say "Okay".
So time matters.
Same answer sounds so different for different age braket. I have known a girl who was 5 years old. If anyone asked her "How are you sarah?", she would say "Couldn't be better..". We used to laugh thinking she must be making fun of grown ups. After 14 years, I met her in the subway, asked "How are you Sarah?", she still replied "Couldn't be better..". It didn't sound odd.