Branding the Vista Logon Screen
Enterprises could easily brand the logon screen for Windows XP by using Registry Editor to set the HKEY_USERS\.DEFAULT\Control Panel\Desktop\Wallpaper registry value to C:\Windows\System32\.bmp where .bmp is some company background image they want to appear underneath the logon box. In Vista however, there is no Microsoft-supported way of doing this.
Want to know more about Vista internals and how to administer Vista in enterprise environments? Get the Windows Vista Resource Kit, Second Edition from Microsoft Press.