I installed DNN on our IIS and everything worked fine and DNN is working. Anyways I now saw in the connection string that it is obviously running on a SQL Express server, although there should be our corporate MSSQL Server 2005:

<add key="SiteSqlServer" value="Data Source=.\SQLExpress;AttachDbFilename=|DataDirectory|Database.mdf;Integrated Security=True;User Instance=True" />

Does someone how to switch that to our SQL Server 2005? The site is not in production already, so there will be no pages etc. lost if we start from a blank database.

Jens,

you need a) to specify  the correct SQL Server instance .\SQLExpress is a local SQL Server Express installation, except there had been an upgrade from Express to full version in the past. You may explore correct version using management studio or server administration.

b) if using SQL Server express, you shouldn't use database.mdf, because you cannot access database the same time using SSMS, please read http://www.dotnetnuke.com/Community/Blogs/tabid/825/EntryId/1202/DNN-SQLExpress-SQL-Server-Management-Studio.aspx 

c) web.config holds by default templates for file using database and database.mdf - make sure you don't look at the commented <!-- ...  --> strings.

d) always make sure that both connection string (in connection strings section and legacy version in the next section) are identical.

Gerard,

server needs to be specified by "(servername)" only, if the default instance is used, or "servername\sqlserverinstancename" else.

If you are on (localhost), you may use "." as servername, e.g. ".\SQLExpress". SQL instance shall be the same as used in SSMS

besides, make sure, both SQL client and server do support TCP/IP authentication and remote authentication is turned on on SQL server as well.

HTH