Updates.  Windows has been prone to update problems ever since Windows update was launched.  Windows 8 is the worst about it of all.  Once Win8 gets stuck it’s stuck.  The process for recovery is quite laborious.  In many cases it is because WU installed an update either out of order(which isn’t supposed to be allowed) or it installs the update but fails to notify itself of the installation(which is bad).  I have worked on several machines that have this issue and the fix is to either one by one back out of the machines installed updates, try a system restore, or reformat the machine.  I can do any of these but usually if there is a good backup series going i’ll try to go back on a backup(i am not a fan of system restore) but if that fails i’ll slick the machine.  A/v suites inside of windows 8 tend to be problematic as well.  Combine the two with the flakiness of WU and you often have a recipe for WU headaches.  A/V suites do not usually start the mess but they typically make the situation worse in this regard.