Internet Explorer (IE) has a neat feature--Autocomplete--that remembers previous answers to password prompts and forms and automatically fills them in on subsequent visits. If you want to clear this information, perform the following steps:

1. Start IE.
2. From the Tools menu, select Internet Options.
3. Select the Content tab.
4. Click Autocomplete.
5. Click "Clear Forms" if you want to remove all information except passwords. Click "Clear Passwords" to clear only passwords.
6. Click OK to the confirmation.
7. Close all dialog boxes.

The actual information is stored in a protected portion of the registry under HKEY_CURRENT_USER\SOFTWARE\Microsoft\Protected Storage System Provider\\[SID of the user\]\Data. However, a user can't view the information (although as an administrator, you can change the ACL via regedt32.exe to have a look).