title: Exceptions from Excel API after downgrading from 2013 to 2010
body: <p>After downgrading from Excel 2013 to 2010 (i.e. removing 2013 and installing 2010) when I try to access the Excel OM from my add-in like this:</p>
  
  <pre><code>if (this.Application.ActiveWorkbook == null)
  {
      return;
  }
  </code></pre>
  
  <p>I get the following Exception:    </p>
  
  <blockquote>
    <p>Unable to cast COM object of type 'System.__ComObject' to interface
    type 'Microsoft.Office.Interop.Excel._Application'. This operation
    failed because the QueryInterface call on the COM component for the
    interface with IID '{000208D5-0000-0000-C000-000000000046}' failed due
    to the following error: Library not registered. (Exception from
    HRESULT: 0x8002801D (TYPE_E_LIBNOTREGISTERED)).</p>
  </blockquote>
  
  <p>I have taken the following steps to resolve the issue but none have worked:</p>
  
  <ul>
  <li>Repaired the office installation</li>
  <li>Uninstalled and reinstalled</li>
  <li>Cleared out all office 15 references from the registry</li>
  <li>Re-registered Excel ("C:\Program Files (x86)\Microsoft Office\Office14\excel.exe" /o)</li>
  </ul>
  
  <p>However, nothing seems to work. Is there something I'm missing or do I actually have to wipe the machine?</p>
  
  <p>EDIT:
  My guess is that there is still come conflict in the registry but I have no idea where...</p>
  
  <p>Thanks in adv!</p>