If you can't obtain it by digging through your system registry (there's a thread about that floating about somewhere), I'm afraid the only option you have is to purchase a new physical copy (if you can find one), or get it via digital download (with the attendant issues of mods potentially not working).
Machinima maker? Using The Movies? iClone? Moviestorm? Any other engine? Then check out TMUnderground at (http://www.tmunderground.com/)