I am a Christian. Naturally, I have non-Christian friends. And you know what? That's fine. I'm not going to preach, because I do not want to upset anyone, but I am going to say that, rather than disappointed, she is most likely just sad. Not because you have a different religion as she is scared and sad that you may end up in hell. In Christianity, we believe that the only way to heaven is through accepting Jesus. I myself get very sad and scared when I think about my best friend going to hell, so I can relate. I just wanted you to understand this first and foremost, if you didn't already.
If your friend is being annoying and irritating and is nagging you about converting into a Christian, by all means, please do not listen to her. That is not how she should be acting and she should respect what you believe. If she is curious, or asking about your beliefs in a way that doesn't make you annoyed or angry, answer her questions. She might want to understand better.
And finally, if the fighting drags on and gets too irritating, tell her how you feel, or change the subject before the fight can even start. Just remember to be gentle about it, because it's no use losing a friend over something like this.
My best friend is very Christian, but questions me for believing in something different. I love my friend, but I hate our bickering. I respect her religion, but whenever it's brought up I feel like she's disappointed with me. Anyway we can get past this?