Every version of Excel can open Excel files created in older versions of Excel; you don't need any other software for that. The same applies to Word, Access, PowerPoint & Outlook.
Presumably, then, you're referring to an addin; that would have to be re-written to work as desired with the Ribbon. There are also addins available that give the same kinds of toolbars, etc., that Office 2003 & earlier had, but I would recommend against using any of those; the old toolbars, etc. aren't coming back and you should by now have learnt to use the Ribbon.