As suggested by Microsoft
Originally Posted by Microsoft
You can run IIS 6.0 in one of two modes: worker process isolation mode or IIS 5.0 isolation mode. Worker process isolation mode is the default mode, that is, the mode that the server runs in on a clean installation. You can use IIS 5.0 isolation mode to run applications developed for older versions of IIS that are determined to be incompatible with worker process isolation mode.
This means you will not be able to take advantage of worker process isolation and the other features of worker process isolation mode.