According to Microsoft regarding named vs default
Client applications connect to an instance of Microsoft SQL Server 2005
to work with a SQL Server database.
Each SQL Server instance is made up of
a distinct set of services that can
have unique settings. The directory
structure, registry structure, and
service name all reflect the specific
instance name you identify during
setup.An instance is either the default, unnamed instance, or it is a named
instance. When SQL Server 2005 is in
installed in the default instance, it
does not require a client to specify
the name of the instance to make a
connection. The client only has to
know the server name.A named instance is identified by the network name of the computer plus
the instance name that you specify
during installation. The client must
specify both the server name and the
instance name when connecting.By default, SQL Server installs in the default instance unless you
specify an instance name. SQL Server
Express, however, always installs in a
named instance unless you force a
default installation during setup.