Thanks for this as DCOM was really the problem for me too. I had a
slightly different setup, though, so worth documenting that
separately.

I really do have Excel 2007 as part of the Office 2007 suite on
Windows Vista. I started getting this issue after I changed the
Identity that Microsoft Excel 2007 ran as to a specific user (for
development purposes). To resolve it I had to change the Identity
back to "Launching User" and magically this problem has gone away
again.

Hope that added info helps someone.