<p>When I try to create a Excel 2007 Workbook project, in Visual Studio 2008, I get the following errormessage:</p>

<blockquote>
  <p>Project cannot be created because the "Excel Visual Studio Design-Time Adaptor Add-in" is not working correctly. Excel might have disabled the add-in or made it inactive, or all add-ins might be disabled in your Trust Center settings. Check the add-in status in Excel Options. If the add-in is active and enabled, reinstall or repair Visual Studio Tools for Office.</p>
</blockquote>

<p>I have verified that the add-in is not inactive or disabled and I have tried to repair and uninstall/reinstall VSTO several times.</p>

<p>What to do?</p>

Just in case you haven't checked the trust settings, this
MSDN page
describes how to.
I came across this on a MSDN forum:
For the repair to work, you'll have to
  run the VS Command Prompt with Admin
  Priviledges (right click, run as
  administrator). Then run this command
  line:
C:>AddinUtil
  -AddInRoot:"%CommonProgramFiles%\Microsoft
  Shared\VSTA\AppInfoDocument" -Rebuild
If it still doesn't work, you may also
  have a corrupt pipleline store, which
  can be fixed like this:
C:>AddinUtil
  -PipelineRoot:"%CommonProgramFiles%\Microsoft
  Shared\VSTA\Pipeline" -Rebuild
You'll get one warning from that
  command but that is expected.
Failing that, I think you will have to run a repair on the Visual Studio 2008 installation (put the installation DVD in a drive, and select the repair option).