killerholic IS correct. win ME, 98, 95, 3.1 are all written over the DOS codebase. That is why you still have real-mode DOS in all of them. NT/2000/XP are on the NT codebase. EDIT: wow, just noticed how old this thread is. <{POST_SNAPBACK}> It's correct that there's a DOS codebase for backwards compatibility, but it's not just an "operating environment" that appears to be a DOS application like Windows 3x and earlier was. Windows 95 and later has 32-bit components. Windows 9x don't require DOS drivers (except HIMEM.SYS, which is required during the loading process before the 32-bit memory management takes over) unlike Windows 3x and earlier.