Client cannot access SQL 2005 Express Server

I've installed SQL 2005 Express, enabled local and Remote connections but my client app cannot connect to the SQL server. I'm getting access denied or SQL does not exists. Any idea's on how to resolve this issue?

Perhaps these articles will help:

Configuration -Configure SQL Server 2005 to allow remote connections
http://support.microsoft.com/default.aspx?scid=kb;EN-US;914277
http://blogs.msdn.com/sqlexpress/archive/2005/05/05/415084.aspx

Configuration -Connect to SQL Express from "downlevel clients"
http://blogs.msdn.com/sqlexpress/archive/2004/07/23/192044.aspx

Configuration -Connect to SQL Express and ‘Stay Connected’
http://betav.com/blog/billva/2006/06/getting_and_staying_connected.html