According to the Installation Docs for DNN 6.1.2 here, Microsoft .NET Framework V3.5sp1 (required for DNN 5.2+) is required to run.

During my installation on a bare-metal box with just Windows Server 2k8R2, SQL2008 Express, and IIS7.5, and .Net Framework 4.0 I found that during the execution on fhe DNN installer I would get random errors that indicated that the system couldn't read the sources.  Eventually after confiriming all security settings were right, I found the issue.

 I was using the DefaultAppPool, and by default, that apppool uses .NET Framework 2.0 and not 3.5 or higher.  As such of course the app won't run as expected.  As soon as I modified the Advanced properties of the app pool and made sure it was using the .NET 4.0 framework it fired up like a charm. 

 If this was previously stated or laid out in the forum my apologies for duplication.  However, in my search of the forums I didn't find a specific indication of such.  I hope this will help someone out there. 

Make it a Great Day

D-Man