Fast User Switching, the Welcome Screen and <Control>+<Alt>+<Delete>
First of all, let's start with a brief explanation of Windows' multi-layered sand-boxing and separation of logins and the user's desktop.
The basic structure is thus:
- Windows kernel and Object Manager
- "Sessions"
- Window Stations
- Desktops
- Your desktop 'n' stuff.
I.e. the kernel can maintain any number of "sessions", all of which can have any number of window stations, all of which can have any number of desktops, all of which can have, well, the windows themselves.
Certain things are sand-boxed at different levels. Named kernel objects (e.g. events, critical sections, etc.) are stored in a session-specific location. Atoms and the clipboard, for example, are part of the Window Station, where as hooks are part of the Desktop. You can only send messages between processes with the same desktop, too.
All the sessions use the same setup:
-
WinSta0 (Window Station)
- Winlogon (Desktop) AKA "secure desktop"
- Default (Desktop)
The login screen, locked dialog and <Control>+<Alt>+<Delete> dialog all run in WinSta0\Winlogon. All your applications run in WinSta0\Default.
It should be noted that applications can create their own Window Stations and Desktops; indeed, my main system currently has 5 extra Window Stations (created by services, like Task Scheduler).
Windows NT and 2000
With Windows NT and 2000, things were simple. Session 0 was the one and only local "interactive" session (the one attached to the physical display device, physical input devices, etc.). Remote Desktop created other sessions for each remote login, as you might expect.
Windows XP
With Windows XP, two things were done to make life easier for the home user - Fast User Switching, and the Welcome Screen.
As you might have guessed, Fast User Switching utilises the "sessions" layer, allowing completely isolated user sessions. Adding this local disconnect/connection required enough work on the session system for an entire book, but it's not relevant here - what is, though, is that any session may be connected to the local physical devices.
Now that the system can do this magic, a UI is needed. Enter the Welcome Screen. The Welcome Screen (logonui.exe) runs in the secure desktop (WinSta0\Winlogon) when necessary, such as when a user logs off or locks the computer. It is interesting to note that the actual process is started by these events and does not run all the time.
-
Initial system
- Active desktop: \Sessions\0\WinSta0\Winlogon
- Welcome Screen running. No user's logged in.
-
Initial login
- Active desktop: \Sessions\0\WinSta0\Default
- No secure desktop processes.
At this point, I will point out that Start > Log off > Switch Users does exactly the same thing as <Win>+L (Lock).
-
Locking initial login
- Active desktop: \Sessions\0\WinSta0\Winlogon
- Welcome Screen running. One user logged in.
-
Second login
- Active desktop: \Sessions\1\WinSta0\Default
- Welcome Screen creates new session and connects local system to it. No secure desktop processes afterwards.
-
Locking second login
- Active desktop: \Sessions\1\WinSta0\Winlogon
- Welcome Screen running. Two users logged in.
This is where things get more interesting. The Welcome Screen can unlock either session, but is currently still connected to the second login. This has a bearing on, for example, sound - which is still connected, so your music will keep playing when you lock the computer (as it always has done). If you unlock the initial login, however, it will be disconnected.
-
Unlock initial login
- Active desktop: \Sessions\0\WinSta0\Default
- Welcome Screen connects to session 0, and unlocks. No secure desktop processes afterwards.
Just for fun, you can also disconnect the local session (from Task Manager, f.e.), which starts a new session just for the Welcome Screen:
-
Disconnect local session
- Active desktop: \Sessions\2\WinSta0\Winlogon
- Welcome Screen running. Two users logged in.
There are two UI points that change other than the login screen:
- Security (<Control>+<Alt>+<Delete>) dialog.
- Locked Workstation dialog.
Most people will see that the Locked Workstation dialog is counter-productive when Fast User Switching is enabled, and thus you get the Welcome Screen instead of it.
The Security dialog is a slightly different story. Winlogon uses GINA modules show various login UI. This includes all the classic login dialogs, the locked workstation dialog and the security dialog. The GINA module can be replaced with a 3rd-party one, such as NetWare's login screens and there is no telling what a 3rd-party security dialog might do. This means that the <Control>+<Alt>+<Delete> behaviour must still exist for non-local situations, and it makes sense (for consistency) to keep showing it when you also have the classic login UI.
In other words, although being in a domain or other network environment does mean sacrificing the Welcome Screen, there is no hard and fast reason for not having the <Control>+<Alt>+<Delete> dialog when using the Welcome Screen. It's just nicer not too. ;-)
Windows Vista
With Windows Vista, things change again. GINA modules are gone, presumably replaced with something else. This is what allows it to present the Welcome Screen for domain setups, and a pretty version of the classic security dialog has been added too. Nice one.
Disclaimer: all reasons behind design decisions, why X does Y, etc. are all based on 2nd-hand data, debugging and intelligent guesswork. I make no claims to them being strictly accurate.