How to delete the username ?

I am using winXP + IE 6, For all the websites I've visited that need username and password to logon. It remembered the username(although I didn't choose to remeber the password). so sometimes when I clicked on the logon textbox, a list of username will pops up from which I can choose. This is convienent, but risky as well, I'd like to know how to make the username list disapprear? I tried to clear the cookie. But didn't work.
thanks!

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