Back in the day, there were two different development sources for Windows:
Windows 3.0 - 3.1 - WfW 3.11 - Windows 95 - 98 - Me .. these versions could trace their roots back to MS-DOS 1.0. They were pseudo-32 bit operating systems running on a 16-bit kernel that was an extension to an 8-bit DOS system.
Then there was Windows NT, originally released as 3.1, then 3.5 - 3.51 - 4.0 - 2000 - XP - Vista - Windows 7. This was a fundamentally different beast than the DOS-based versions, as it was designed to run on different hardware (the original Windows NT ran on MIPS as well as Intel platforms). The NTFS file system with access control (ACLs), preemptive multitasking and other "mainframe" features were included - the original Windows NT was designed by the same brains that originally designed Digital Equipment's VMS (one of the best operating systems, IMHO).
Comparing Windows 98 to Windows XP is like comparing a Model T Ford to a Ford GT. They both came from the same company, but that's about all they have in common. Windows 98 "looked" like Windows NT 4.0, but under the covers, they were two totally different systems.
The DOS-based Windows line died with Windows Me (no great loss!). The development of Windows NT, which started in the early 90's, continues into Windows 7. I'm sure that there is still quite a bit of original Windows NT 3.1 code in the Windows 7 kernel!
In other words, there hasn't been a "revolutionary" change in Microsoft operating systems since the early 90's. Most, if not all, of the changes from Windows NT 3.1 up thru Windows 7 have been feature-enhancing (e.g. adding Internet Exploder), bug-fixing (patching security holes due to sloppy programming in the past) or cosmetics (wow, a sexy new Aero-interface, woohoo
).