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this is in the registry and cannot be edited that easily.
i did however find something on M$'s website,
QUOTE
SUMMARY
It is possible to hide the user name of the last logged on user in the logon dialog box in Windows NT.
MORE INFORMATION
WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
Start Regedt32.exe and locate the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
Add a REG_SZ value named DontDisplayLastUserName. Set the data value to 1 to enable the new feature. A data value of 0 disables the feature.
The Last Username Display can also be hidden without editing the Registry directly if you have the Windows NT Resource Kit.
1. Click Start\ProgramsResource Kit 4.0
2. Click Configuration\C2 Configuration
3. Double click the line that reads "Last Username Display"
4. Place a check in Hide the last username to login.
5. Click Secure then OK.
if you need anymore help with anything, feel free to ask