Provided are a computer readable storage medium, computer apparatus, and method for securely managing the execution of screen rendering instructions in a host operating system and virtual machine. A first rendering instruction hooking section is set to a first mode to hook a screen rendering instruction issued by a virtual machine application in a virtual machine. A second rendering instruction hooking section is set to a second mode to hook instructions issued by the virtual machine application. The hooked screen rendering instruction issued by the virtual machine application are encrypted in response to the setting of the first mode to produce illegible output. The hooked screen rendering instruction issued by the virtual machine application are encrypted in response to the setting of the second mode. The encrypted hooked screen rendering instruction encrypted in the second mode are issued to a host operating system to decrypt.