title: All Visual Studio 2013 builds are now failing
body: <p>Every single project I open in Visual Studio 2013 is now all of a sudden giving me the following error:</p>
  
  <blockquote>
    <p>EverySingleProject.csproj : error  : The default XML namespace of the project must be the MSBuild XML namespace. If the project is authored in the MSBuild 2003 format, please add xmlns="http://schemas.microsoft.com/developer/msbuild/2003" to the  element. If the project has been authored in the old 1.0 or 1.2 format, please convert it to MSBuild 2003 format.  C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets</p>
  </blockquote>
  
  <p>This is in the file:</p>
  
  <blockquote>
    <p>Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003"</p>
  </blockquote>
  
  <p>Why is this failing?</p>