I think the first step in troubleshooting your issue is to go through web.config and make sure everything is OK with the SQL connection string. Check to see if the server name, database name, user name and password are OK.

Check web.config carefully - there are at least 2 lines that mention SQL

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Joao Beltrao
IT Manager
Desejo Sem Limites Multimedia
PORTUGAL

Well, I've tried now on a brand newly installed Windows Server 2003 SP2, and the same thing happens, it keeps saying that it can not connect to SQL server, even though I know there is no problems in accessing SQL server. However, I've tried all server string combinations in web.config file like (local), (localhost), (SERVER\INSTANCE), (SERVER), (INSTANCE), SERVER\INSTANCE, etc. but non of these worked. I keep getting ASP.NET event 1309 from source: ASP.NET 2.0.50727.0 and here is the event itself. (it's always the same)

Event code: 3005
Event message: An unhandled exception has occurred.
Event time: 21-03-2007 13:49:56
Event time (UTC): 21-03-2007 12:49:56
Event ID: bf7c4b4caa8f45b1abba63680535cb58
Event sequence: 2
Event occurrence: 1
Event detail code: 0
 
Application information:
    Application domain: /LM/W3SVC/1460427975/Root-4-128189549801846548
    Trust level: Full
    Application Virtual Path: /
    Application Path: D:\Inetpub\www21itpro\
    Machine name: ARTEMIS
 
Process information:
    Process ID: 3652
    Process name: w3wp.exe
    Account name: NT AUTHORITY\NETWORK SERVICE
 
Exception information:
    Exception type: SqlException
    Exception message: An error has occurred while establishing a connection to the server.  When connecting to SQL Server 2005, this failure may be caused by the fact that under the default settings SQL Server does not allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
 
Request information:
    Request URL: http://www.domain.com/Install/Install.aspx
    Request path: /Install/Install.aspx
    User host address: XXX.XXX.XXX.XXX
    User: 
    Is authenticated: False
    Authentication Type: 
    Thread account name: NT AUTHORITY\NETWORK SERVICE
 
Thread information:
    Thread ID: 7
    Thread account name: NT AUTHORITY\NETWORK SERVICE
    Is impersonating: False
    Stack trace:    at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject)
   at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
   at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
   at System.Data.SqlClient.SqlConnection.Open()
   at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(String connectionString, CommandType commandType, String commandText, SqlParameter[] commandParameters)
   at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(String connectionString, CommandType commandType, String commandText)
   at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(String connectionString, String spName, Object[] parameterValues)
   at DotNetNuke.Data.SqlDataProvider.GetPortals()
   at DotNetNuke.Entities.Portals.PortalController.GetPortals()
   at DotNetNuke.Common.Global.CacheMappedDirectory()
   at DotNetNuke.Common.Global.Application_Start(Object Sender, EventArgs E)