Data source needs to be SQLServerBox\SQLEXPRESS
You are using just .\SQLEXPRESS so it's going to look for
SQL Server locally - on the IIS box. If you just copied the
web site to another server and Express, the databases are
still on the other box, that indicates it is likely not
pointing to the correct server.
If you didn't change anything from the defaults on the box
where Express is installed, you will also need to enable
remote connections and make sure the browser service is
running.
And finally, if windows firewall is running on the XP box
where Express is installed, add an exception for
sqlservr.exe

-Sue

On Fri, 27 Oct 2006 14:56:01 -0700, Jim
<Jim@discussions.microsoft.com> wrote:

>The connection string is:
>
><add name="ASPNETDBConnectionString" connectionString="Data
>Source=.\SQLEXPRESS;AttachDbFilename=|DataDirecto ry|\ASPNETDB.MDF;Integrated
>Security=True;User Instance=True"
> providerName="System.Data.SqlClient" />
>
>"Jim" wrote:
>
>> I've build a web application on a Windows XP computer that has SQL Server
>> Express edition installed on it. I then copied the web application to a
>> Windows 2003 Server box. When trying to run the application an error occurs:
>>
>>
>> provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance
>> Specified
>>
>> Why does this error exist and how can it be fixed?
>>
>>
>>