SQL Server 2008: enable login to sa user

1. Login using Windows Authentication and ".\SQLExpress" as Server Name
2. Change server authentication mode - Right click on root, choose Properties, from Security tab select "SQL Server and Windows Authentication mode", click OK 
3. Set sa password - Navigate to Security > Logins > sa, right click on it, choose Properties, from General tab set the Password (don't close the window) 
4. Grant permission - Go to Status tab, make sure the Grant and Enabled radiobuttons are chosen, click OK 
5. Restart SQLEXPRESS service from your local services

Source:

http://stackoverflow.com/questions/3781365/how-to-change-sa-password-in-sql-server-2008-express