struggling to use Windows authentication with SSRS

When attempting to use Windows authentication in an SSRS datasource like so:

Data Source=ThatServiceThatRunsOnPortFourteen,14;Initial Catalog=Whatever;Trusted_Connection=Yes

 
 

One may get this error in running a report:

To use the values in the connection string, you must configure the unattended report processing account for the report server.

 
 

http://msdn.microsoft.com/en-us/library/ms156302(v=sql.90).aspx touches on Configuring an Account for Unattended Report Processing