If the users are required to type login and password to an IIS Web Server which allows anonymous login, it is quite possible that the Metabase is corrupted or out of sync with the NT Authentication System. The solve this problem, we will need to synchronize both IWAM_Machine account and IUSR_Machine accounts between IIS Metabase and NT Authentication.

Below are the detailed steps:

1. get a command prompt by typing CMD in the run box.
2. go to c:\inetpub\adminscripts folder, if IIS is installed on c:\inetpub as default.
3. At the command prompt, type: cscript synciwam.vbs -v
(Check if the problem is resolved. If not, please go on to Synchronize IUSR_Machine account.)
4. modify the script adsutil.vbs in c:\inetpub\adminscripts folder to change
the line "IsSecureProperty = True" to "IsSecureProperty = False"
5. At the command prompt, type: cscript adsutil.vbs get w3svc/anonymoususerpass
(You will get a clear text password for IUSR_Machine)
6. Type this password into web server anonymous account, you will need to do it twice to confirm.