title: Excel Addin not appearing on some machines
body: <p>I am having some difficulties with installing an Excel addin at my client. I have created an installer for the addin using the instructions at <a href="http://www.clear-lines.com/blog/post/create-excel-2007-vsto-add-in-msi-installation.aspx" rel="nofollow">http://www.clear-lines.com/blog/post/create-excel-2007-vsto-add-in-msi-installation.aspx</a>, and it has been successful for the majority of users. The installer was run as an admin and this seemed to work ok. However for a small proportion of users the installation was unsuccessful - the registry entries are created but the addin does not load when Excel starts and does not appear in the addins list. Trying to add it manually from the installation folder gives an error that the addin is not valid.</p>
  
  <p>To complicate matters slightly, the client uses both Vista and XP (although all installations of Excel are 2007). None of the XP machines have installed the addin successfully, whereas most (but not all) of the Vista machines have been successful. </p>
  
  <p>Has anybody had any experience of this, or might be able to point out where I am going wrong?</p>
  
  <p>Many thanks in advance,</p>
  
  <p>Rob</p>