Targeted my project in Visual 2010 to .Net 4.0 but the system still looks for the dll 'System.Core version 2.0.5.0'

This is mainly for future me, who will at some point come back to this question, and none of the current answers will solve your problem (hey future me!)

My issue was a nuget package referencing System.Core 2.0.5.0 so I had no control over the reference.

To get System.Core 2.0.5.0 installed on the machine, which already had .NET 4.0 installed and patched, I installed the Portable Class Library.

To install the Portable Class Library tools on a build machine without installing Visual Studio 2012, download the Portable Library Tools, and save the download file (PortableLibraryTools.exe) on your computer. Run the installation program from a Command Prompt window, and include the /buildmachine switch on the command line.

I had a problem similar to this that only occurred on windows XP/Server 2003 while Windows 7/Server 2008 and later worked fine. After a long time of looking I ran into this article:

http://code.google.com/p/autofac/wiki/FrequentlyAskedQuestions

It suggested downloading the following hotfix

http://support.microsoft.com/kb/2468871

After installing that everything worked fine! Maybe give that a try and see if it helps.

Probably you included reference to some libraries belonging to .Net 2.0 and they depends on System.Core 2.0. So you need to check every reference's version number included in your project.