I encountered this error and posted a prior question on EE. Here is the link
My previous EE question on this error
Rather than explaining everything again please refer to the prior question.
The suggestion I received and accepted were to add the MS Office reference and make sure that the entire Office suite was installed on the machine.
I finally got back to the client. I discovered that the MS Office 14.0 reference was already present and that the entire MS 2010 Office suite was installed on the machine.
I am still getting exactly the same error. Any other ideas?