Windows 8 and 8.1 do not include .NET 3.5 by default. If you can’t enable .NET 3.5 via the Turn Windows features on or off (for example on a domain {SBS 2008} where this function is disabled) then simply insert a Windows 8 or 8.1 installation disc (or mount an image), run an elevated command prompt and run the following:
Dism /online /enable-feature /featurename:NetFx3 /All /Source:F:\sources\sxs /LimitAccess
Where F: is the drive with the installation disc/mounted image