I was not able to launch zwift after windows updates on 1/18/16. Needed to disable Windows Defender to get past the Preparing for Update loop. I hope this helps someone.
Just add C:\Program Files (x86)\Zwift to your windows defender exclusions directory, then you can leave your malware protection enabled.
I had trouble with Zwift crashing at the Orange Entry page after updating Windows 10 to #1709. Adding the Exception to Windows Defender solved the problem for me.
Just add C:\Program Files (x86)\Zwift to your windows defender exclusions directory,