Have you configured SQL 2005 to accept remote connections?
Are you sure that the instance name is correct?
Are you able to connect to the SQL 2005 instance from the other server?
Is the SQL Browser running?
This <a href=”http://itknowledgeexchange.techtarget.com/sql-server/steps-to-troubleshoot-connections-to-your-sql-server/”>blog post</a> shows how to troubleshoot connections to a named instance.