Error 0x800F0906 while installing .Net 3.5 on Windows 8

There's an item at the bottom of this library article that details the steps you should take:

If installing the .NET Framework 3.5 on demand or enabling it in Control Panel fails, you may get error message 0x800f0906: "Windows couldn't connect to the Internet to download necessary files. Make sure that you're connected to the Internet, and click Retry to try again." This message may be displayed for the following reasons:

1/ Your computer is not connected to the Internet. Please connect, and then retry the operation.

If you are unable to connect your computer to the Internet, you can enable the .NET Framework 3.5 by using the installation media (ISO image or DVD) you installed Windows 8 Release Preview from. Mount the Windows 8 Release Preview installation media and type the following command from an elevated command prompt:

Dism /online /enable-feature /featurename:NetFx3 /All /Source:x:\sources\sxs /LimitAccess 

where x: is the drive letter for the Windows 8 Release Preview ISO image or DVD.

2/ Your administrator has configured your computer to use Windows Server Update Services (WSUS) instead of Windows Update for servicing. Please ask your administrator to enable the policy to use Windows Update instead of WSUS.