Recently I have changed the network from Domain to Workgroup.
Then I removed the domain controller.
Today I just realized that I could not create a database on my PC's SQL Server 2008 Express.
It said that I have no permission to do it. Hey, I have administrative permissions!!
Then I figured out that the local users were not registered on the SQL Server 2008. Besides I did not want to think of complex password and used "Windows Authentication Mode".
Here's what I did.
1.Change the SQL server's authentication mode to "Mixed"
ref: http://blogs.wankuma.com/route/archive/2008/06/12/143047.aspx (Japanese site)
1a. Open Registry Editor and open "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL10.SQLEXPRESS\MSSQLServer"
1b. Change the value of "LoginMode" to "2" which means Mixed Mode.
2.Restart the SQL Server in "Single User Mode"
ref: http://blog.sqlauthority.com/2009/02/10/sql-server-start-sql-server-instance-in-single-user-mode/
2a.Open Server Contoller applet and modify the startup parameter of SQL Server Service.
2b.add '-m;' before the other parameters.
2c.Then restart the SQL Server Service.
Now you can log in to the SQL server with your windows account and have all permission you need!!
So I first added myself as localuser to SQL Server's user.
Then I changed the password of 'sa'. Ah, don't forget to enabled the user as well.
Thank you Pinal Dave and WANKUMA! You saved my day!!!