This is a problem caused by a software patch that makes changes to the Windows 8 registry editor. It sometimes deactivates the system activation process and prevent the Windows Update from contacting Windows server for necessary updates installation, as I have observed in some computers. There are other causes like those mentioned in Windows official webpage .
Image of Windows Update_800B0101 at Windows Help and Support in Windows 8
At the Windows Update, from the Control Panel, the procedure to solve the problem is not listed in Windows Help and Support nor is there any reference directly pointing to this particular error. It prevents Windows defender from updates while it shows the system is protected from malicious software.
Image of Windows Defender fails to update in Windows 8
If you are having this problem with your system, there is step-by-step instruction provided here in getting your system back to function properly as it was before.
Image of System Restore in Windows 8
STEP 1: Do a system restore to previous time before the trouble of Windows Update 800B0101. On how to do system restore in Windows 8 visit this LINK .
Image of Checking for updates in Windows 8
STEP 2: After the system restore, go to your Windows 'Control Panel' click 'Windows Update'. To the right-hand side of your Windows Update dialogue click 'Check for updates'.
Image of Updates downloading in Windows 8
STEP 3: Download and Install necessary updates. If the Windows asked you to restart the system, simply restart your system for changes to take effect.
STEP 4: If you are still experiencing the error and don't have a restore point in your system, follow this LINK