Can't Deauthorize Audible Account

I'm having trouble using Audible because of an inability to deauthorize a former account. The situation is this. About a year ago I set up an audible account in itunes, but never used it. I completely forgot about it and then one day a month ago I decided to try audible again. Forgetting I had authorized an account in itunes already I just set up a new one at the website and bought some books.

Now the problem is I can't download those books because the former account is already authorized with itunes and can't be deauthorized without the user info, which I don't remember. I need a way of deauthorizing the old audible account in itunes without knowing the username and password.

I'm on Windows XP with iTunes 7.6.2.9.