Windows 9x has process isolation, but has no concept of permissions thus any process can do anything that is provided by the OS API (which includes reading and

Windows 9x has process isolation, but has no concept of permissions thus any process can do anything that is provided by the OS API (which includes reading and writing memory of other processes, injecting code into kernel and such things). This means that it is very unlikely for misbehaving process to compromise the integrity of OS itself or other processes (modulo the fact that such process can confuse some other OS component, eg. window manager, such that the system becomes essentially unusable).