Or you could just do what I do:
Acquaintance: Oh, you do computers? Can you fix my Windows?
Me: No, I don't work on computers outside of my job.
Nearly everyone who is an acquaintance has gone through that conversation with me at some point in time or another, and it's never been a problem. Most people just shrug and say, "Oh, well can you recommend somebody who does?" I think most computer professionals are afraid of how people will respond to them being blunt about that sort of thing, but the reality is that most people (in my experience, at least*) are perfectly accepting and understanding about it.
*I suppose, though, that it might be worth noting that although I'm typically the shy / quiet / not terribly social type, I am tall, in good shape, and have a deep voice, so when I do make myself stand up straight, look people in the eye, and be assertive and direct, they tend to respond how I'd like them to. YMMV if you're closer to the stereotypical 'geek', especially if you can't learn to make yourself use body language properly.