On my Vista machine at work, applications and the SQL Server Management Console work fine connecting to SQL Server 2005. Sometimes they are ok for weeks at a time, sometime for hours and then they stop connecting.

I've tried everything to get it to work including the installation of SPII and running the user provisioning tool without any luck. The only way to fix it was to restart.

The Error:

Connections are refused with the standard error message:

Cannot connect to SERVER_NAME\INSTANCE_NAME

------------------------------ ADDITIONAL INFORMATION:

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified) (Microsoft SQL Server, Error: -1)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&EvtSrc=MSSQLServer&EvtID=-1&LinkId=20476

The Fix:

Stop and restart the Sql Server Browser, Sql Server integration, SQL Server Active Directory Helper services.

Works like a charm.