See www.microsoft.com/windows, books, and magazine articles for this
sort of information. You may have to dig back into the archives.
Basically, each version of the OS needs the type/spec of hardware
typically on sale in the market place during the time of it's relevance.
From a software perspective, are all differnt products. XP and 2000
have "roots" from Windows NT, and ME and 98 have roots from Windows 95.