Why does Imports System not give access to Windows namespace in vbc.exe?

Question

I manage an application that allows the users to automate tasks by writing their own VB code. The user code is compiled using the VBCodeProvider and invoked against the running instance of the application. We've been doing this for a few years now starting with .NET 2.

Traditionally, we have imported the System namespace in the compiler settings so users wouldn't have to write System. all the time. When we went to .NET 4, however, we found that statements like Windows.Forms.Form wouldn't compile anymore. The error was "Type 'Windows.Forms.Form' is not defined." This is odd because other namespaces work. IO.Stream and Reflection.Assembly do not have an error without the System at the beginning.

I've created a simple example. I've put the below code into a file. Then I compiled this file with vbc.exe from both the .NET2 and .NET4 directories. The 2 version works fine. The 4 version will not compile unless you comment out the variable f2.

Imports System

Public Class MyClassName

    Public Shared Sub Main
        'this works in v2 and v4
        Dim f As New System.Windows.Forms.Form
        f.ShowDialog

        'this does not work in v4
        Dim f2 As New Windows.Forms.Form
        f2.ShowDialog
    End Sub

End Class

Does anyone know how to get this to compile in vbc.exe version 4? And before you say "Just tell the users to type System.Windows.Forms" I will agree that it would be great if they would do that, but users do what users do and I have to work it out.

UPDATE:

I've found that the Windows.Foundation.Diagnostics namespace is causing a collision with the abbreviated use of Window.Forms. Is there any way to hide this namespace from my compilation? Visual Studio 2010 does not have the same conflict, so it must be getting around it somehow.