Some versions of Microsoft Office will not run alongside previous installations.

Mac

Office for Mac will allow you to run multiple versions. You will notice it may change your file associations and set itself up as default, making it look as if your previous copy of Office isn't there anymore. If you check in Finder > Applications list, you should see both versions.

Windows

Office for Windows will walk you through the process of uninstalling any previous versions during the install of the new one.