error: "Failed to generate a user instance of SQL Server due to a failure in starting the process for the user instance"

delete the C:\Documents and Settings\username\Local Settings\Application Data\Microsoft\Microsoft SQL Server Data\SQLEXPRESS directory।

http://www.differentpla.net/content/2006/01/failed-to-generate-a-user-instance-of-sql-server
http://blog.krisvandermast.com/FailedToGenerateAUserInstanceOfSQLServerDueToAFailureInStartingTheProcessForTheUserInstanceTheConnectionWillBeClosed.aspx