"Windows failed to install the following update with error 0x80070643: Definition Update for Windows Defender"
In the last few days the automatic update of the definition file for Windows Defender has been failing in my Surface 2. Troubleshooting the issue I found that you can install the definition manually even in a Windows RT device by visiting this site and downloading the proper file, in this case of a Surface 2 you should download the one under Windows Defender for Windows 8.1 ARM.
Update: at the end the issue was resolved by deleting the c:\Windows\temp folder. The whole procedure is explained here.