After installing Microsoft Update KB3133977 for Windows 7, some users may encounter a "Secure Boot Violation", or see this message "Secure Boot Violation, invalid signature detected. Check Secure policy in setup.", which makes the system fail to boot into the operating system.
To protect user's systems from malware attacks, ASUS motherboards implement the Microsoft Secure Boot feature by default. This feature performs a legal loader check to boot into the OS. With the update of KB3133977, Microsoft have changed Secure Boot policies, hence, the system detects inconsistent OS loader keys which can result in boot failure.
The solution to this issue is to change UEFI Secure Boot settings as outlined in the steps below; this will allow the system to boot into the operating system successfully.
Step 1: Enter the UEFI and navigate to Advanced Mode Menu->Boot->Secure Boot
Step 2: Change “OS type” to “Other OS”
Step 3: Press F10 to Save the changes and reboot
Step 4: Check the UEFI Advanced Menu->Boot->Secure Boot, and confirm whether the “Platform Key (PK) State” is switched to be “Unloaded”.
Step 5: Exit the UEFI, and the system will now boot normally.
Here's an example video: https://www.youtube.com/watch?v=n9rNv7xOrKg