GraFowler,
One will use DOS if you boot from it the other will use Windows PE if you boot from it. The Diagnostics CD also has the full Windows version.
Windows PE is the lightweight Preinstallation Environment for Windows. It, as the full Windows, is highly dependant upon drivers being present. It is used to provide basic testing with generic drivers.
The DOS diagnostics on the diagnostics CD talk directly to the hardware with no intervening drivers.
It is most usefull to use the different version as a team. The DOS to test the hardware directly and the windows versions to test the system holistically, testing hardware and driver interaction.
-Fred
