I suspect its either auto complete or saved passwords in your browser. Most likey autocomplete. It seems removing a single entry isn't straight forward but you can clear the whole lot (if you don't use auto fill yourself elsewhere)
See here
Note if you type in your own address and password on you troublesome page IE should ask you if you want to update the saved details?
Regards
Peter Gillespie
Edit: Actually this method may be the easiest: When you start typing in a Web form with AutoComplete enabled, you will see a list of previous entries for the same field that begin with what you've typed so far. You can select one to avoid retyping it. To see all stored entries for a field, click in that field and press the Down Arrow key. To remove an entry that is erroneous, outdated, or private, highlight it using the Down Arrow (not the mouse) and press Del.
