two colleagues, how to stop it getting messy

I've been in my current job for nearly a year now and for much of that a girl I work with has quite obviously liked me. Although not much younger than me, I am a lot more independent and mature than her and without meaning to sound like a dick it would be like going out with a child. As a colleague and friend I get on with her very well, but any further would be disastrous. I think she thinks I'm being shy. I'm not. I don't know how I am meant to reject somebody who has not asked me out. Do I stop replying to texts? Refuse to go places with her? I want neither, but I also don't want to hurt her by leading her on. Then there's a complication. There is another girl at work I do think would be a good match, but have refrained from approaching the subject. I could well get knocked back, but then I might not. It is not my fear of rejection though but my desire not to hurt somebody I have no desire of building a sexual relationship with by approaching somebody that I do. Guess I'm more talking to myself here, but hoping for advice anyway.

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