Often things remembered in autofill dropdowns can be removed by double-clicking the input field or typing a letter to make the browser show the list, and then use the cursor keys to go down in the list and highlight it. Next, depending on the browser and operating system, hit something like Delete, or Shift-Delete, or Command-Delete, or Shift-Backspace to remove it from the list.
I also assume IE6 has some settings in Tools, Internet Options that is related to remembering user names and passwords. Next to that option you'll find a button to show you what has been remembered. But IE6 being so old, I doubt anyone can give you the exact location to that option.