Dot dot ad continuum....I've had this several times. One PC I stopped and rebooted the system several times and it finally kicked through. 2 other PC's I used the Fix It tool from MS to repair something gone haywire in the registry. I've also heard sometimes clearing out the internet cache helps.
I recently had the same problem and solved it by shutting down windows and then restarting it. A mere reboot was not enough. Only at shutdown it started installing updates and then the next boot it found the next set up updates.