Navigate to the key below and change the value of "DontDisplayLastUserName" to 1 to prevent the username from being displayed in the login dialog box.

This is recommended in a network environment.  Because if a hacker were to see this, they would be half way to getting into the system.  All they have to do now is guess the password.  If the last logged on username isn't displayed then they would have to guess the usernames and passwords.

Key:[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon]
Value Name: DontDisplayLastUserName
Data Type: REG_SZ
Data:(1=enable, 0=disable)