Some say that the KB3004394 update prevents MSE from installing. So you could try uninstalling KB3004394 update if you have it installed. http://answers.microsoft.com/en-us/protect/forum/mse-protect_start/mse-error-0x8004ff91/17f5227a-01bf-47b8-92a2-4cb087aaa1e0?page=8 edit: in fact, this last major Microsoft Tuesday update appears to really have screwed things up for some people. MS pulled that KB3004394. http://www.bleepingcomputer.com/forums/t/559332/windows-update-kb3004394-triggers-error-messages/page-2