Referencing a .Net 2 Dll in .Net 4

I have 2 DLL's referenced in my project, both were written in .Net 2 and my application is in .Net 4.

I am developing the application in an XP environment using VS2010, when I debug, or execute the exe file in this environment it works perfectly. But when I debug or execute the exe in a Windows 7 environment the application just closes as soon as it opens. There is nothing in the event log and no exception is thrown. I have added the following to my app.config for the mixed assemblies exception I received :

<startup useLegacyV2RuntimeActivationPolicy="true">

This is what the DLL's reference :

using System.Runtime.CompilerServices;

using System.Runtime.InteropServices;

using Microsoft.VisualC;

Any help would be appreciated.<o:p></o:p>