With the introduction of Windows 8 and Windows RT, Microsoft splits its flagship OS into two distinct flavors. Examine the most important contrasts.
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Windows 8, like previous versions of Microsoft's PC operating systems, will ship on new computers and will sell as a standalone product to do-it-yourself upgraders. Not so for Windows RT. According to a recent blog post by Microsoft's Mike Angiulo, "Windows RT software will not be sold or distributed independent of a new Windows RT PC, just as you would expect from a consumer electronics device that relies on unique and integrated pairings of hardware and software."
In other words, you'll be able to upgrade a Windows 7 PC to Windows 8, but not to Windows RT.