I believe that Windows 8 only comes with .NET 4 active and that version is the first one that isn't backwards-compatible like the previous ones were.
According to a Google search older versions of it should be available to install in Control Panel/Programs "Turn Windows functions on or off" or you can always check online. Sorry I'm not too familiar with 8 yet. I joined Windows 8 forums but haven't read much on there as of yet.
As Windows 8 is (very) beta we don't officially support it anyway.