This happened to me once too, but the deleting build directories advice fixed it. Specifically, you just need to delete the bin\Debug and bld\Debug folders in your projects. Their contents will be regenerated by Visual Studio when you rebuild. I assume that this is only one project since it’s a Hello World app; otherwise I would ask if you deleted build directories from all projects in your solution.
You can also try running “Clean Solution” from the BUILD menu in Visual Studio.
I’m sorry…it’s horrible if this is happening on a clean install as you describe.