With IIS 4.0 you have the option of running an application in a separate process from IIS. To accomplish this, select the Run in separate memory space box in the property page for a directory, or by using the AppCreate method exposed by the IIS Admin Objects.
If you isolate an application, any ASP scripts and ISAPI DLLs stored on the physical directories included within the application boundary will be run in an isolated process. The following diagram illustrates the difference between an isolated application and an application run in the same server process as IIS.