Because chipsets have changed a lot between MS DOS 6.2 and Windows Vista. I assume that the the different Windows profiles in Parallels are because there are subtle changes in chipset drivers that need to be emulated. Parallels emulates an Intel i815 logic board. Intel supplies slightly different drivers for that chipset depending on whether one is running Windows 95, Windows 98, etc.
Plus, each Windows version interacts with the logic board (emulated or otherwise) in a slightly different way. This is yet another reason for the different versions within Parallels. This may also be a the reason that Apple's Boot Camp will only work with Windows XP and Vista, but not any prior versions of Windows.
Additionally, each Parallels "profile" represents a slightly different BIOS configuration tuned to the specific OS and its capabilities.
Think about it this, Parallels (and possibly VMWare's Fusion) is trying to emulate an Intel i815 chipset on an (at least) Intel 945 chipset platform. There are likely significant differences in how they each chose to approach this task.
Last edited: Aug 24, 2007