It's not worth it, especially when you start to develop feelings.
I've been there, and this girl had no problem trying to get me jealous by flirting with other men as they walk past my office every day. You think you can get over that **** quickly, but it doesn't work out that way. You can't really focus on your work, you get hot under the collar as you have to put up with her presence/games, and your mind will start to wander as to what/who she's doing at the moment (and in whose office).
I tried to treat her like any one of the employees, but whenever I did so, she'd bring up past stuff that happened between us, and make it personal. The next day, she'd go cold on me and not even behave in a friendly, professional manner.
She was hot, funny, and a great lay, but I would never have bothered to even look at her had I know the **** I was going to endure at work (for 2 additional years) as a result of getting with her. It was a living hell.
The day she told me she was leaving the company was the one of the happiest moments of my professional life.
My advice: Focus on your work.