Handling Excel 2007 file

i m getting error Unable to cast COM object of type 'System.__ComObject' to interface type 'Microsoft.Office.Interop.Excel.Application'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{000208D5-0000-0000-C000-000000000046}' failed due to the following error: Library not registered. (Exception from HRESULT: 0x8002801D (TYPE_E_LIBNOTREGISTERED)).