When you try to log on to a Microsoft Windows-based computer by using a user name that contains an “at” sign (@), you receive the following error message:

The system could not log you on. Make sure your user name and domain are correct, then type your password again.

When you try to log on to a Windows Vista-based computer by using an account that does not have administrative permissions, you briefly receive a “Welcome” message, and then you receive a “Logging off” message. Additionally, you may receive a warning in the Application Windows log that resembles the following:

The Windows logon process has failed to spawn a user application. Application name: . Command line parameters: c:\windows\system32\userinit.exe.

Source: Winlogon 
Event ID: 4006 
Level: Warning 

This issue does not occur if you log on to the computer by using an account that has administrative permissions.

Consider the following scenarios:

• An additional monitor is connected to a Windows Vista-based desktop computer. The additional monitor is designated as the primary display (the main monitor). The computer goes into hibernation or goes to sleep. This additional monitor is disconnected. The computer resumes from sleep or from hibernation.
• An external monitor is connected to a portable Windows Vista-based computer. The additional monitor is designated as the primary display (the main monitor). The built-in screen is designated as the secondary display. The computer goes into hibernation or goes to sleep. This external monitor is disconnected. The computer resumes from sleep or from hibernation.

In these scenarios, you cannot see the logon screen. Therefore, you cannot log on to the computer.