MS SQL Server 2008 User Account Settings

To create an assembly with either external_access or unsafe permissions, you should have the assembly strong name signed or Authenticode signed with a certificate, or you should have the database configured with the set trustworthy on statement. If the certificate server has failed, then it may prevent the assembly from being signed by a certificate. You can use the alter database Sales set trustworthy on statement to ensure the success of the with PERMISSION_SET = EXTERNAL_ACCESS Statement.

Adding an assembly to the DB:
The classes in the assembly cannot contain finalizer methods. A finalizer method is a method that runs when an object of a class is removed from memory. Classes within an assembly associated with SQL Server 2008 do not need finalizer methods, because the SQL Server runtime handles finalization.
The EXTERNAL_ access permission set allows the assembly to access the registry and environmental variables, along with external system resources such as the file system, the external network, and Web services.

Changing startup account for SQL Server:You should use the SQL Server Configuration Manager to change the startup account. Using SQL Server Configuration Manger is the most appropriate method of changing the startup account. The startup account is used for running the SQL Server services. If you change the startup account, the services must be restarted. Without restarting, the older startup account remains tied to the SQL Server services. After you restart
the services, the new account becomes active to run the SQL Server services. You can invoke the SQL Server Configuration Manager form

Start > All Programs > Microsoft SQL Server 2008 > Configuration Tools > SQL Server Configuration Manager. 

SQL Server Configuration Manager is used for configuring services, network protocols, and network configuration on SQL Server 2008. You can also use the Services snap-in to change the SQL Server startup account.
Logon triggers are used to audit and control server sessions. You can track login activity and restrict logins to SQL Server.
When a database is attached to a SQL Server, the database is not initially trusted and cannot access resources beyond the scope of the database. If you explicitly mark the database trustworthy, then you can access resources beyond the scope of the database.