1. A first computer in a virtual computer system comprising a plurality of computers for executing a program and a storage system connected to the computers via a network, the first computer comprising: a virtualization mechanism which generates at least one virtual computer to which a physical CPU and a physical memory of the first computer are divided and assigned, wherein the virtualization mechanism in the first computer executes at least one of a first judgment and a second judgment when a request for activating the first virtual computer of the at least one virtual computer is received, wherein the first judgment is for judging whether or not the activation of the first virtual computer is prohibited based on first information stored in a memory of the virtualization mechanism, wherein the second judgment is for judging whether or not an identifier included in a logic I/O adapter device assigned to the first virtual computer is invalid based on second information stored in the memory of the virtualization mechanism, wherein, when the first virtual computer is requested to be activated, the activation of the first virtual computer is prohibited if a setting for prohibiting the activation is valid in the first judgment or if the identifier is registered to be an invalid identifier in the second judgment, and wherein the virtualization mechanism includes: a management table for storing a correspondence between the first virtual computer and a setting for prohibiting activation of the first virtual computer, the setting being either valid or invalid, and a virtual computer activation unit for executing the first judgment for checking the setting for prohibiting activation of the first virtual computer based on the management table when the virtual mechanism accepts an activation request for the first virtual computer, and for prohibiting the activation of the first virtual computer if the setting for prohibiting the activation is set to be valid.