VBA Excel call to c# dll via Visual Studio 2012

I have created a dll lib via the following:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace VBAprocs
{
    public class fileSys
    {
        public string getDirectories()
        {
            return "Directory A";
        }
    }
}

using the COM interop facilities  ie checking both the "Make assembly COM-Visible" and the "Register for COM InterOp" boxes on the project property pages [Visual Studio 2012 Express Edition]
However when I try and build the class library, I get the following message regardless of the fact that I am executing from an account with administrative privileges. What do I need to do to get the dll library registered?

"Cannot register assembly "D:\VS_Development\VBAprocs\VBAprocs\bin\Debug\VBAprocs.dll"  - access denied. Please make sure you're running the application as Administrator. Access to the registry key 'HKEY_CLASSES_ROOT\VBAprocs.fileSys' is denied."