Defrag won't touch a file that contains physical errors - eg one of the disc sectors has gone bad, and it usually won't give you an error message either.
Check the disc for errors (easiest way is in explorer, right click on disc, select properties then tools then check for errors)
All that said and done, a large Dr W file may only be indicative of some other problem or infestation, so be prepared to carry on.
Have you run tools like spybot and hijack this?