How to earn back the trust of a girl?

So there is a girl I've been deeply, madly, truly into for a very long time. I think that my ex has contacted her and told her some lies or something and now she thinks that I had sex with my ex or whatever... we talked about it yesterday and she seemed pretty mad(in her own way, she never shows it)... I only think of my ex as a friend (thought of killing her a couple of times, yeh). I think somebody(perhaps my ex) is just trying to ruin the relationship that might be ahead of me and this girl.. how do I prove that I am only a friend with my ex to that girl and what should I do? I really don't want to lose her, she is like the most important person for me. And ofc I didn't have sex with my ex for ages... I want noone but that girl I'm into.
P.S. She finally got onto msn.

I feel like bumping.
Bump be it.

Make a huge, public show of getting rid of your ex. Embarrass her, call her a destructive liar in front of her friends and tell her that while you once thought you could be friends, you've now realized what a spiteful, rotten person she is for trying to ruin the special love you have for this sweet, wonderful girl of yours.

Do this immediately.