While error code 0x80070426 usually refers to a non-genuine copy of Windows, in this case, the error refers to MSE not working correctly. When error 0x80070426 appears, it leaves your computer completely vulnerable as Windows Defender doesn’t even start up and MSE doesn’t work properly.
The solution to this problem is to simply uninstall Microsoft Security Essentials, allowing Windows Defender to do its job and protect your computer.
The following are all of the steps you need to follow in order to fix error 0x80070426.
Hold the Windows Key and Press R. Type taskmgr in the run dialog. Locate the Windows Defender Service (WinDefend), right click on it and choose Stop.
s: appuals