I've wrestled with KB2909921 for two months now and tried every fix in the book and all of the MS toolkits to no avail. The update would install then reconfigure to about 93%, fail and revert. Tweaking.com (all in one) has done the trick. Actually the Windows Update got to where it would not work either with code 80070424. After running the All-In-One the update window was fixed and KB2909921 installed without problem. There are several folks in cyber-land having the same problem. I hurried to the donate site and made a small offering.
You are overworked and underpaid. Good job!
