Yes, if you use office automation (as what you're doing now), you need to add a reference to the Microsoft Office xx Object Library (where xx is the version number) found in the COM tab. Keep in mind that anyone who uses your application will also need MS Office of the same (or higher) version installed in their machine.
Another way to do this is to use ADO.Net... Just read your excel file into a datatable then write the table to an xml file... This requires no MS Office installed in the machine.