The application has failed to start because config.dll was not found. Reinstalling the application may fix the problem

Question

I have developed a program in c#.net 4. When the application is executed directly by clicking exe file in the debug folder then an MDI form is open. When I click on a button in the MDI form to open another form then it shows the following error:

"The application has failed to start because config.dll was not found. Reinstalling the application may fix the problem"

But when the application is run from the source code in VS 2010 then no error is shown.
I didn't use any config.dll in my application.
I couldn't figure out the problem?