1. A system for executing a program, the system comprising: a virtual machine monitor which divides an operating system (OS) into a plurality of domains having different trust levels including at least one root domain; and a trust-management module which is included in the root domain and which periodically measures a trust level of an application program currently being executed in the OS, wherein the virtual machine monitor executes the application program in one of the plurality of domains in consideration of the measured trust level of the application program.