Since the technique below was for the Windows 8 Consumer Preview, I wanted to share how you can accomplish this in the RTM release of Windows 8, from my reply to a commenter on the post:
For the RTM release, you can use “Turn Windows Features On or Off” in settings (just open the Search charm for settings (Win+W), and type “Turn Windows”, and you should see it at the top of the second column). Once you’ve got that dialog up, the first option available will be “.NET Framework 3.5 (includes .NET 2.0 and 3.0)”. Check the checkbox, click OK, and you should be all set.
Original post follows:
Jaime Rodriguez provides a helpful tip for folks who need to install .NET 3.5 in the Windows 8 Consumer Preview (in addition to Windows Live Writer, I’ve found that the highly-useful utility Mouse without Borders also requires .NET 3.5, so I’ve used this technique on a couple of occasions):
I was installing Live writer and got a prompt to install the .NET Framework 3.5, I agreed to do that and got a “following feature could not be installed. Error: 0x800F0906.
Fortunately, I still had the ISO from when I installed consumer preview , so I used the local sources.
If you see the error above, try this from an elevated command prompt.
dism.exe /online /enable-feature /featurename:NetFX3 /Source:h:\sources\sxs
where “h” is the drive where you have your win8 installation files (or where the iso is mounted, replace the path appropriately ).
Happy Windows 8 coding!