How did Windows 8 let you install Office 2000? As for upgrading files, you mean like the documents you created in that dinosaur? You just insert the DVD for Office 2010, it will see the old version, state that, you select Customize to "Run all from My Computer".
It will not remove the old dinosaur version of Office, it will just sit there on the disk alongside. If you wish to remove 2000, you have to do it from "Programs and Features".