Usually what happens when you install a new version of an application, the new version will look for any previous versions and delete all files and contents, after that it starts a fresh new installation which means your office 2003 is now gone. Here is an example if you were to try and install an older version of DirectX without deleting the previous one then you would get a message saying you already have the latest or higher version of DirectX and it would ask you if you still want to continue or quit the setup. So, reinstalling would be your only option unless there is a recovery software of deleted files which I think there is.