Please download the .jar version of the installer (not the Installer-Win variant) from our official downloads page: http://files.minecraftforge.net
Open the folder containing the downloaded file in Windows Explorer.
Hold down shift and right-click an empty space, then select "Open command window here".
In the new window type java -Djava.net.preferIPv4Stack=true -jar [name of the installer], replacing [name of the installer] with the complete file name of the installer, including the .jar file extension. Then hit enter.
If that does not work, respond to this post providing the installer log file ([name of the installer].log) that is created alongside the installer you downloaded.