The UNO does not use the FTDI chip...
Follow the steps here to disable driver signature enforcement on Win8:
http://arduino.cc/forum/index.php/topic,94651.msg727588.html#msg727588Then follow these steps to install the UNO drivers:
Right click the Unknown Device in Device Manager
Select Update Driver Software
Select Browse my computer for software
Click Browse button
Select the
Drivers folder under the location of your Arduino software installation. (Do not select the FTDI folder inside the Drivers folder)
Click OK
Click Next