http://msdn2.microsoft.com/en-us/library/ms365941.aspx
The SQL Server Database Engine and SQL Server Agent run on a computer as a service, using credentials that are initially provided during setup. If the SQL Server is running under a domain account and the password for that account is changed, the password used by SQL Server must be updated to the new password. If the password is not updated SQL Server may lose access to some domain resources and if SQL Server stops, the service will not restart until the password is updated. After changing a domain password (using Windows), change the password of the accounts used by the Database Engine and for SQL Server Agent by using SQL Server Configuration Manager.
To change SQL Server Authentication passwords, see Password Expired.
To change the password used by the SQL Server service
1.Click the Start button, point to All Programs, point to Microsoft SQL Server 2005, point to Configuration Tools, and then click SQL Server Configuration Manager.
2.In SQL Server Configuration Manager, click SQL Server 2005 Services.
3.In the details pane, right-click SQL Server (<instancename>), and then click Properties.
4.In the SQL Server (<instancename>) Properties dialog box, on the Log On tab, for the account listed in the Account Name box, type the new password in the Password and Confirm Password boxes, and then click OK.
5.The password takes effect immediately, without restarting SQL Server.