Launcher Error -2146697211 after Windows 10 Upgrade.
http://puu.sh/jr0uj/6b4226a8f5.png
Finally Fixed it, the culprit is indeed Windows Defender. However, if you had my issue, you couldn't disable Windows Defender because it was not not being used since you have Avast (or another Anti Virus) program running, so you must disable your current Anti Virus first then you can disable Windows Defender.
This is how I fixed it, I use Avast so I will use that as reference. Right click Avast and disable it.
http://puu.sh/jC1ep/436e33e6ee.jpg
Then open your windows search bar and type Windows Defender and go to the settings.
http://puu.sh/jC1kw/d7d985ee62.png
Now the Windows Defender settings shouldn't be greyed out, if it is, you may need to reboot your computer. Now you can TURN OFF EVERYTHING IN WINDOWS DEFENDER.
http://puu.sh/jC1rg/f6c827269c.png
Now enjoy.
http://puu.sh/jC1KK/a0494a67e4.jpg
DO NOT FORGET TO TURN YOUR AVAST BACK ON!!!!!!!!
AVG User - Windows 10 issues
Not sure what happened between my original installation of Windows 7 and subsequent upgrade to Windows 10, but I had this issue as well.
Tried adding the files as exceptions, no joy. Paused AVG for 10 minutes and had no issues getting to the sign-in screen. Definite anti-virus issues. Not sure what the workaround is at the moment, will advise if I find anything further.