System.Windows.Input doesn't exist?
<p>I have <code>using System.Windows.Input;</code> at the top of my program, but it gives me an error saying that:</p>

<blockquote>
  <p>The type or namespace name 'Input' does not exist in the namespace 'System.Windows' (are you missing an assembly reference?)</p>
</blockquote>

<p>And when I let IntelliSense popup on the period right after <code>Windows</code> it only lists <code>Forms</code> as a valid option. I'm using .NET framework 4.0 in Visual C# 2010 Express...</p>

<p>How can I fix this problem?</p>
