On a computer that is running Windows Vista or Windows Server 2008, you enable the “Run logon scripts synchronously” Group Policy. When you try to log on, the Welcome screen may be displayed for 30 seconds. Then, the logon script interacts with you before the logon script process continues. For example, the script prompts you to confirm a dialog box.

Note: The “Run logon scripts synchronously” Group Policy setting is in the following location in the Group Policy Management Console:
Local Computer Policy\User Configuration\Administrative Templates\System\Logon\Logonff

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.