In one computer system, causing the second virtual machine, which executes antivirus software for detecting and removing the virus, to monitor at least one first virtual machine that is created on the computer and execute one or more application program, periodically storing a state of the first virtual machine as snapshot, suspending the first virtual machine from which the virus is detected if the antivirus software executed on the second virtual machine detects the virus, and restoring the first virtual machine at a state of a point in time when the snapshot is stored by using the snapshot of the suspended first virtual machine.