Running Nonnative Applications in Windows 2000 Professional
Windows 2000 provides support for nonnative applications by emulating the environments in which they are designed to be processed. This support is provided through environment subsystems and Virtual DOS Machines (VDMs) . Each environment runs as a separate user-mode process, protected from errors in the other environments. Failure in one subsystem does not disable another subsystem or the core operating system. Applications are also user-mode processes, so they cannot hinder the other environments or the kernel-mode processes.
Except for the Microsoft Win32 subsystem, which is the native environment of Windows 2000, each environment is optional and is loaded only when a client application needs its services.