After you install update 3133977 on a Windows 7 x64-based system that includes an ASUS-based main board, the system does not start, and it generates a Secure Boot error on the ASUS BIOS screen. This problem occurs because ASUS allowed the main board to enable the Secure Boot process even though Windows 7 does not support this feature.
Microsoft also has a solution:
The Secure Boot feature is supported in Windows 10. To learn more about the security advantages of this feature and about the upgrade path from Windows 7 to Windows 10, go to the following Windows website: www.microsoft.com/windows
To be fair, this problem is the fault of ASUS rather than Microsoft, but switching the update from optional to recommended is going to be what causes people trouble.