Just installed an UNO on my new Windows 8 machine to find that signed drivers are required.
What eventually worked was to use the SEGGER JLink CDC UART port driver instead. You can get it from the Atmel Betaware site:
http://www.atmel.no/beta_ware/usb/AtmelUSB.exe
It just points to the Microsoft usbser.sys driver that comes with Windows 8, but is digitally signed.
I happened to need to install that newer USB driver because the supplied AVR Jungo driver was not signed. I don't think you have to install the entire AVR Studio 6 package to make this work.
I just thought I'd share this little victory in the hopes it will help some other Windows 8 users.
Please let me know if it works for you.
Dale