By default ADO.NET, including ADO.NET 2.0, uses Named Pipes to connect to SQL Server. (Including SQL Server 2005) However, in some environments where there are firewalls and other network appliances between say your ASP.NET web client or other Windows Form client and the database you might find that Named Pipes don't work.

If you need to use TCP/IP to connect to SQL Server instead of Named Pipes then the solution is to use the connection string parameter "Network Library".  You simply add "Network Library=dbmssocn" to your connection string.  This is defined in the MS KB article at: http://support.microsoft.com/default.aspx?scid=kb;en-us;Q315159