From Microsoft:
The Msgina.dll file interacts with the Winlogon service to provide the Welcome screen and the Fast User Switching feature. Third-party software can replace the current Graphical Identification and Authentication data with their own Graphical Identification and Authentication data. The Fast User Switching feature and the Microsoft Welcome screen are not available if you are using a third-party product that replaces the Microsoft Graphical Identification and Authentication file (msgina.dll).
To restore the default Windows logon interface, remove the following string value from the registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\GinaDLL
After you do so, Windows uses the default Graphical Identification and Authentication file (Msgina.dll). (Me: do a hard drive search for the msgina.dll file before you try this. Also, export a copy of the entire Registry to a known location before editing the Registry.)
Fast User Switching is also not enabled if the available RAM is less than 120 megabytes (MB). Systems that have Shared Video Memory (where a portion of the system's RAM is used for video display memory) may reserve 8 MB or more for use as video RAM. If the system has a total of 128 MB of RAM installed, the total available RAM may be 120 MB or less after video memory is reserved. This will cause Fast User Switching to be disabled.
The Fast User Switching Compatibility service uses Shsvcs.dll. Without this file, the service cannot be started.