I seem to have SDC caught in a loop. The progress meter alternates between Downloading GC2TwilightOfTheArnor.exe_ and extracting GC2TwilightOfTheArnor.exe (without the underscore).
A couple of things flash by too quickly to observe in the middle of the loop, and when the Downloading phase is active, the %complete seems to be totaling all the loops. It is over 5700% now, so I think I should give up for the moment.
That happened to me, too, but when I ran SDC as an Administrator, it went away.