Hi,
The Feature you are refferring to is called as Auto Complete.
You can enable / Disable this feature in IE 8 under
Tools > Internet Options > Content > Auto Complete > Settings
This is where you can enable / disable the autofill / autocomplete option.
In order to clear the already saved passwords Tools > Browsing History > Delete. In here you would find the option to clear Saved Uids and passwords.
¬JNSV