once when i was asked this by my boss i replied 'fine thankyou', my boss then came back with 'just fine?' the next time she asked i replied with 'ok thankyou' it seems 'ok' is not good enough either, so the next day i replied 'couldn't be better thankyou!' and was met with good! so it seems that if asked how you are the reply is 'couldn't be better'
usually i simply say fine thankyou as this way no explination is usually needed. though a couple of good friends i will always tell them the truth from exhausted to ok to aaaagh! because they really do care and will always help me sort things out if i need someone to talk to.