Yeah, I've complained to co-workers enough that most of them know to not ask or immediately apologize before I have a chance to respond. For customers, I generally ignore the question and ask how I may assist them. I've actually offended three people in the last decade for doing that. "Oh, ok. I guess you're FINE then!" or something bitchy along those lines. In some cases, I will respond with various bits of nonsense, such as
"I'm what you kids would call 'groovy!'"
"I still have all nine of my fingers."
"I can still taste the fish oil I had this morning."
"I didn't do it! Wait, what did you say?"
"Mur. Also... mur."
"What? What?! WHAT?! This better be about pizza.
"I hugged a dog. He farted. My day's complete."
"I want to tell you I'm doing well, but that'll jinx it, and my day will be ruined, but I can't say I'm not doing well, because I might convince myself of that, and both of us would be worse off for the response, so I'm going to say a whole bunch of stuff really fast and we can both get back to doing productive stuff, like not burning more of the chicken I left in the fryer earlier. I'm sorry for the smell."
"I hate everything. You?"