This can be all sorts of things.
But my first guess would be that the .NET framework isn't installed on the pc you copied the files to.
Or the incorrect version is installed.
When you go to your project properties under the compile tab you have a button 'advanced compile options' there you can see what framework you'll need.
Try creating a setup project for your client program and use that to install on the client pc.
The setup will check if you have the correct version of the .NET framework and if you haven't will download it and install it.
Visual Studio Windows Application Setup Project[
^]
Its for a bit older version of visual studio but you'll get the idea
Tip: build your project in release mode (will improve performance)