If you’re already familiar with screensavers, then you know that XP allows you to require users to submit their password to unlock the computer once the screensaver is running. By default, the Unlock Computer dialog box will supply the name of the current user, requiring only a password to be provided to get back to the desktop. This configuration suffers from the same issue as displaying the last username in the Log On to Windows dialog box, since half the required information to access the system is already provided.
To disable XP from displaying the username and require users to provide both a valid username and password to exit the screensaver, yet another Registry edit does the trick. Open Regedit and access the HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Winlogon key and create a new REG_DWORD value called ForceUnlockLogon. Setting this to a value of 0 requires the user to submit a password only, while a value of 1 requires both the username and password to be resubmitted.