I had the same problem, and appear to have found the culprit in my case.
I recently had some malware which disabled both windows update and the BITS services. This was evident when entering the windows update screen in the control panel only to find that windows update was unavailable as the services were not running (sound familiar?). Attempt to activate the services and they are not there!
To resolve a google search later on windows update services not being available, downloaded (from somewhere on the www) the registry keys for BITS and wuauserv, installed and restart and the updater is now happily downloading.
Thus perhaps check the BITS and Win Update services?