I have Surface Pro 4 with Win 10. I initially started out using Windows Defender but decided to switch to Norton 360. The issue that I was experiencing is that Windows Defender continues to try to install definitions. The following link will describe the solution to this problem... http://answers.microsoft.com/en-us/windows/forum/windows_10-update/how-to-stop-windows-defender-definitions-update/bf355a0a-66d6-403e-a7d0-ae334f44cb45