This is partially correct. the files for SxS folder are included in Windows 8 / Windows 2012 but are on the ISO (DVD) not the install.wim as they were in win7, so do not get installed to HDD. To add them to the WIM so you don't need windows update you can mount the install.wim i.e. dism /mount-wim /wimfile:"E:\sources\install.wim" /mountdir:"C:\wimmount" /index:1 then run dism /image:C:\wimmount /enable-feature /featurename:NetFX3 /All /source:"<win8 source DVD>:\sources\sxs" then finally unmount WIM dism /unmount-wim /mountdir:C:\wimmount /commit. If you have already installed the image you can insert win8 dvd or source files and run cmd within win8 dism /online /enable-feature /featurename:netfx3 /all /source:"<win8 installation dvd>:\sources\sxs"
techno45