The present invention relates to a computer having an isolated user computing unit for responding to a system seizing attempt by a malicious code and minimizing damage to a system. A computer according to a feature proposed by the present invention comprises: a security management computing unit for managing connected I/O devices and auxiliary memory device unit; and a user computing unit which is isolated from the I/O devices, communicates with the I/O devices via an intercommunication unit responsible for communication between the security management computing unit and the user computing unit, has a separate CPU and memory, and is connected to the security management computing unit. The security management computing unit manages the I/O devices, monitors and restores a system, and monitors and controls the user computing unit, and the user computing unit is isolated from the security management computing unit and executes a user program and a user OS.