Oh, I see. Sorry to misunderstand you.
More fundamentally, DOS/Windows was not built from the ground up as a multiuser, networked OS; Unix was.
They had the opportunity to fix that at least three times that they've bragged about. Windows 9x->WindowsNT, WindowsXP and now Vista. Each time they've said it was going to be a complete rewrite. Each time they end up with the same old mistakes.
Oh, no doubt, I'm not letting Microsoft off the hook for that. Sure, there's a lot of legacy stuff to support, but Apple has managed to change processor architectures twice and move to a completely rewritten OS with a pretty smooth transition each time.