I found the problem, the user that gets used is called 'BUILTIN\Users' and this was not defined in SQL 2005.

Thanks for your help.

Bob
>>Hello all,
>>
>>I am not sure if I should have posted this in Microsoft IIS forum, but I am getting the following error when trying to run my web app:
>>
>>Exception Details: System.Data.SqlClient.SqlException: Login failed for user 'IIS APPPOOL\DefaultAppPool'.
>>
>>
>>Now, on this server I have an instance of SQL 2008 as well as SQL 2005. I get the above error only on SQL 2005. I am running the same web app but I just change the connection string in the web.config to point to a different database.
>>
>>Any ideas why I would get the above error in SQL 2005?
>>
>>TIA
>>
>>Bob
>
>Well, do you have such user defined in SQL Server database?