The fix is rather simple though. You have two choices:
1) Run this command from an Administrator command prompt to tell the machine where to get the install files from:
Dism /online /enable-feature /featurename:NetFx3 /All /Source:D:\sources\sxs /LimitAccess
This assumes your installation media is mounted in your CD/DVD drive under D:\.
2) If you are in a corporate network you can always make a GPO change to take care of it like this:
Create or modify your COMPUTER based group policy to incorporate this setting:
Computer Configuration-->Administrative Templates-->System-->"Specify Settings for optional component installation and component repair".
Enable this setting and make sure and select "Disabled" under "Never attempt to download payload from Windows Update".Select "Enabled" under "Contact Windows Update directly to download repair content instead of Windows Server Update Services (WSUS)".
So essentially this tells the machine to get windows updates from whatever source you have specified in your group policy, but get any repair files or optional components from Windows Update.
Once the change is made you can reboot the machine and if the GPO is applied the installation should continue without any more problems. The great part about this is if you have several machines the problem is fixed and you don't have to worry with this anymore. This assumes you do not have anything blocking machines from getting to Windows Update at the firewall/proxy level.
