1. Press Windows+E (the windows button is between left Ctrl and left Alt).
  2. File Explorer will open (the Windows 8.1 version is pictured, the appearance of yours may be different).

  3. Navigate to the folders listed below and look for EXCEL.EXE in each (## = some number 14 or above)
    C:\Program Files\Microsoft Office\Office##\
    C:\Program Files (x86)\Microsoft Office\Office##\
    C:\Program Files\Microsoft Office ##\ClientX64\Root\Office##\
    C:\Program Files (x86)\Microsoft Office ##\ClientX86\Root\Office##\
  4. You may have more than one of those folders if Microsoft Office has been upgraded several times or if multiple versions are installed but at least one of those folders will have EXCEL.EXE in it if Microsoft Office is installed in the default location.