I recently ran into a problem where I had by mistake checked the Remember password checkbox in Internet Explorer 7 (RC) when visiting a NTLM based website, then I wanted to get back to use my currently logged on user to access this website. There is no way to clear these usernames and passwords using the standard ways in Internet Explorer.
First of all I tried to turn off the Automatic logon only in Intranet Zone and entering a new but faulty password for the user and checking the remember password checkbox. This cleared the old password but after resetting the automatic logon Internet Explorer always asked for my password for that site and I didn't want to enter my current logon information and save the password (this would only ask me for a new password whenever I change it).
After some searching on the Internet I found no other solution than the one I tried above. But after digging into the problem for some time I found out where Internet Explorer 7 RC stores the cached credentials. They are stored in file (Documents and Settings\\Application Data\Microsoft\Credentials\<SID>\Credentials) that I removed (of course after making a backup). Then I rebooted the computer and the automatic logon worked fine.
If you are interested in what usernames and passwords that are stored on your machine you can check out Nir Sofers Password Recovery Utilities.