It happened to me a couple of times to start a customer's PC (fortunately not my own ones) only to see that some updates are pending. Before reaching 100% you get a failure message and a restart. After the restart the machine boots normally to windows after reverting changes. But on your next restart you will see the same issues again, reaching 100%, failure, boot normally and Groundhog Day all over again. I found these 2 solutions that worked for me.

1. Within the Windows environment do the following:
- Press Windows Logo+R to bring up the Run dialog
- Copy and Paste the command "dism.exe /image:C:\ /online /cleanup-image /revertpendingactions"
- You should briefly see the Command Prompt window. Do a restart and check if the pending updates are still there.
- Run a full Windows Update once again.

2. You can remove the file "C:\windows\winsxs\pending.xml". If Windows doesn't allow you to do it for security or privilege reasons then you'll have to do it through the recovery console or take the HDD to another PC with running OS and delete it.
After rebooting into the system, run a full Windows Update once again.

*** I've tried it on Windows 7 (Pro and Ultimate) and I see that Microsoft's Technet website has listed this command as compatible with Windows 8 and 8.1.