When you change modes the sa password is still disabled. Here read this article:http://technet.microsoft.com/en-us/library/ms188670.aspx
If Windows Authentication mode is selected during installation, the sa login is disabled and a password is assigned by setup. If you later change authentication mode to SQL Server and Windows Authentication mode, the sa login remains disabled. To use the sa login, use the ALTER LOGIN statement to enable the sa login and assign a new password. The sa login can only connect to the server by using SQL Server Authentication.