Girls will a lot of time respond positively to you just to avoid conflict and risk hurting your feelings. Us guys have to know how to read those situations. Sometimes that involves us being more direct with our intentions and other times it's backing off to see if they come to you. Since this is a coworker, I'd suggest that you back off and see if she comes to you for your attention(I don't mess with women I work with...less drama).
I agree with JackZero a relationship with a coworker creates a lot of drama and she can complain to her boss about harassment. If you want her then let her come to you, otherwise just try to increase your value through body language and your social skills around the workplace.