6.4 is is installed by default, it is just not associated with files by default if a later version is also installed. So you can assign the file associations to it in its configuration or run it from the Run command, depending on whether you are able or allowed to change anything. If you are able to create a shortcut then it would be useful in the Send To folder for convenience and minimally noticeable change to anyone else. Then just right-click on a media file and choose Send To > Media Player 64 (or whatever you call the shortcut).