@christ_child: Extremely hard. Take the original NT as a template: MS decided to build the new Windows in 1988, started in 1989, and finally brought the first version to market in 1993. And that had significantly fewer features than Windows has today. And if I recall correctly, there were some features outlined in the original NT spec that didn't make it until Windows 2000, 12 years after work started.
MS has significantly reworked major parts of Windows on several occasions: Windows 2000, which was targetted for 1997 release but didn't make it until December 1999 (four years of development), and Windows Vista, which took more than five years (not all directly attributed to its own development, but still). And both of those cases were significantly less work than a full rewrite would have been.
Not to mention that there's absolutely no reason to do so. For every bug or design flaw that you could conceivably fix by starting over, you'll introduce a new one.