i had the same problem...i deleted dtd then ran a full virus scan then restarted the pc and reloaded dtd.
it did happen twice in 2 days but since then (and doing the above twice) it has been fine.
i also deleted all my internet history and cookies etc the second time but that could be totally unrelated.
Thanks Smurf, Just deleted history and cookies and tried downloading again and bingo!!! it's working fine again.
Thanks again