Currently, the .NET Framework 4.5 runs only on Windows 7 and above:
Operating system Version 4.5 Beta
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Windows 8 Consumer Preview √ (see notes)
Windows 7, all editions √
Windows Vista, all editions —
Windows XP Professional, all editions —
While I understand that XP support is being dropped, I'm a bit surprised by the decision not to support Vista. (After all, .NET 4 supported XP and above.)
Is there any indication (press announcements, MSDN blog entries, ...) that more operating systems will be supported when .NET 4.5 is released?
Microsoft updated the their .NET Framework System Requirements page to reflect that .NET 4.5 now officially supports Windows Vista SP2, Windows 7, and Windows 8.
Anyone can vote for making Windows XP supported on .net framework 4.5 here http://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/2723735-make-net-4-5-work-on-any-os-that-supports-4-0
来源：https://stackoverflow.com/questions/9989893/net-framework-4-5-operating-system-requirements-no-vista-and-xp