When telling Acrobat to combine DOC/DOCX or XLS/XLSX files Acrobat must use its PDFMaker.
Acrobat asks the OS to open the file(s) native application (Word or Excel for your situation) and then has PDFMaker output a PDF.
Acrobat then combines these PDF files.
But, Acrobat 9 PDFMaker is not compatible in Windows 7.
You can use the Adobe PDF printer to output a PDF from the Word / Excel files then, as you noted, combine the PDFs.
Acrobat X provided a Windows 7 compatible PDFMaker.
However, Acrobat X has been replaced by Acrobat XI.
So, to do what you desire you'd need to upgrade to Acrobat XI.
The available upgrade paths are here: