Only one windows station is permitted to interact with the user at the console; this is called Winsta0. Under Winsta0 there are three desktops loaded: Winlogon (the logon screen), Default (the user desktop) and Disconnect. All three of these have separate logical displays, which is why your main desktop disappears if you lock the workstation. When you lock the workstation, the display switches from Default to Winlogon and there is no user interaction between the two. In Windows Vista this is even a bit more extreme. When you get a UAC prompt for instance, it takes a screenshot of your Default desktop and then displays it dimmed out behind the UAC window in the foreground. The UAC window is part of the Secure Desktop (new for Vista and similar to the logon desktop) and will not allow you to interact with the Default desktop until you provide input.