John, the ASPNET account is a subset of NETWORK SERVICE account in win 2003... in any case, this is a completely different subject. The connction string will indicate the parameters relating to the connectivity, which in his case, would be to SQL server. He (the original John) says that he has configured his database authentication to mixed mode. Now, he will need to give the ASPNET (or in case of win 2003, NETWORK SERVICE) account Read/Execute permission to Microsoft.Net and Temp directoried that fall under the Windows directory. Both VB.Net and C#.Net use these directories to fetch material and store temp files.

Now, your connection string should look something like this:

<add key="SiteSqlServer" value="Server=MAINSERVER;Database=DotNetNuke;uid=DotNetNukeUser;pwd=password;" />

Let me know if you are still having problems... and don't forget to include screen shots