I'm a fan of the used bike idea. It probably won't be your dream bike, it may have some fit issues, but you'll learn a lot on it (like what you want to change, and what kind of riding you want to do). IMO, that will make purchasing a new bike less stressful, less likely to make fit mistakes, and give you a good starting point without breaking the bank.

Craigslist, yardsales, and bulletin boards are all good resources for used bikes.

Good luck, and let us know how it goes!