Use
CCleaner (install without the Yahoo toolbar) to clear this and other private data.
Use
this guide to set it up, but make sure you DO check AutoComplete, even though the guide says not to.
To prevent this happening in the future, open Internet Explorer (this assumes you are using IE7 and WinXP, as that's what I have). Go to Tools --> Internet Options --> Content.
Under AutoComplete, click Settings.
Untick User names and passwords on forms.
IE will now not remember your passwords.
Hope this helps.