There is a special list in the registry which you can use to prevent user accounts from appearing on Vista’s welcome screen. To add a user account to this list, open the registry editor (regedit.exe) and navigate to:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList]
If SpecialAccounts or UserList does not exist, create it. Create a DWORD registry value for the name of the user that you want to hide and name it the same as the user. Make sure it is set to 0, which will flag the user as being hidden on the welcome screen. If you set it to 1, then it will be visible.
And that’s it!