I concur. "Do you have a girlfriend?" requires a simple, yes or no answer, unless you can throw in a little humor in your response, too. She may be interested or she may just be curious (but if she bothers to ask at all she likely finds you at least slighty attractive). You'll have to judge by the context. For instance, if your involved coworker were to ask you that question after you've been at your job for six months, it is more likely to be just curiousity. If the cute girl you've just met at the party asks, she's almost certainly interested. You get the point
"Why don't you have a girlfriend?" is a little trickier. If she asks, she definitely likes you, though she may not be looking to start anything with you. You've successfully showed yourself to be a sexual being, but she's going to evaluate your answer to see if she can catalog any flaws from your response. I got asked this question often during my AFC days, and I gave some pretty lame responses. If you have any interest in this girl, DO NOT express frustration with women, talk about wanting to sleep around, or indicate that you haven't had any chances. Saying that your "too busy" may make you look ambitious, but it also tells her that your possibly a workaholic and not very much fun.
Best reaction I've had is from using the line "I haven't been seeing anybody lately to whom I would want to make that kind of commitment..." or some variation of that. (Credit that to a forgotten poster on this site, by the way) This shows that A) Your still active with women even though your single, B) Your selective in regards to woman, and C) that you're open to commitment. A C/F comment is a nice touch, but you might want to follow-up with a serious answer, or she might think your ducking the question