SQL 2008 R2 Named Instance Client Connectivity Issues?
Try disabling your firewall, as it might block port 1434 for SQL Browser
Is the SQL Browser Service Enabled and Started?
http://www.mssqltips.com/sqlservertip/1946/overview-of-the-sql-server-browser-service/
SQL Server 2005+ works differently than 2000 in identifying server instances.
The SQL Browser Service is probably not running.
See here.
In SQL Server 2000, the identification of the server connection endpoints was performed by the SQL Server service. SQL Server 2005 replaces that function with the SQL Server Browser service.